Like many geeks, I'm a fan of a well-designed system. When I look at something and see harmony and cohesion and logical dependencies, I consider it a thing of beauty. I also enjoy a system that's robust, and built to last. It's why "my" edition of D&D will likely always be 3rd edition (in the form of Pathfinder). And contrariwise, it's why I don't think it matters what edition of Warhammer rules are current. Significant pieces of the Warhammer 40,000 design transcend any edition number, and that's a key strength of the system.
That's NOT to say that every edition of Warhammer 40,000 is the same. The 1st edition rules used miniature profiles that would break the game in 10th edition. A Weapon Skill of 5 was great in 1st edition, but it would be dismal in 10th. The 7th edition rules allowed for rolling a 7 and higher on a d6. There are objectively significant differences from edition to edition that make them incompatible with one another. You cannot play a game of 10th edition against an opponent using 9th edition rules. However, a game edition isn't only about the way attributes are assigned values, or even the sequence of the game.
Ignoring market forces, the public release of a new edition of an existing product is a function of correction. When a designer identifies something in a product that doesn't work the way the designer believes is best, then a change gets made. When there are so many changes that new version of a product is no longer compatible with the previous version, the new product is declared a new edition.
What form that process takes in the real world varies from producer to producer. One game publisher (like Wizards of the Coast for D&D and Games Workshop) might celebrate the release of a new edition, while another (like Wizards of the Coast for Magic: The Gathering) might consider game rules a "living document" that evolves with no need for any edition number.
Revision tracking is the obvious reason for edition numbers. Functionally, an edition number impacts 3 aspects of a game: Rules, expansions, and lore.
Early in the first edition of the Warhammer 40,000 rulebook, it's written that if you already know the rules of Warhammer Fantasy, then you can skip to the next chapter. Games Workshop obviously considered the game a thematic variant of their existing game system, Warhammer. 35+ years later, and to an extent the same is true for every new release of Warhammer 40,000. If you've played one edition, you've basically played them all. I'm overstating and over-simplifying things, but the core game loop hasn't changed since, at least, the first edition of Warhammer 40,000 (if not earlier).
There are certainly major changes from edition to edition, but in my experience of buying old editions of rulebooks, it's a matter of making minor adjustments to how your game proceeds rather than re-learning the game as if you'd never played before. As long as you're familiar with the game, you can pick up a new edition quickly because it still relies on the same game loop, and it reuses the same terminology for rules and special rules.
Games Workshop built an infrastructure for its rules, and it doesn't play outside that infrastructure. When they want to deviate from an established system, they make a new game. Warhammer (now called the Old World) was replaced by Age of Sigmar. Cursed City and Blackstone Fortress are boxed games, even though they're set within the Age of Sigmar and 40k universes. Battle for Balin's Tomb is a boxed game even though it's set in Middle-earth. Underworlds is a separate system, even though it's a Sigmar game.
I believe Games Workshop builds a game system exactly once. Editions don't reinvent the system, but revamps it.
Because an edition isn't a reinvention of a game system, much of the content within a gaming expansion isn't affected by an edition change. Game scenarios and even game modes (like Boarding Actions) are mostly usable across editions, because most are written to be independent of rules. Usually, they don't introduce new numbers and they don't alter numbers. When they do alter a number, it's often a "safe" number (with a 6-sided as the foundation, it's pretty obvious what adding or subtracting can do to a roll) and it's expressed in generic terms. You don't add or subtract 1 from an attribute, you "worsen" or "improve" the attribute by 1, which leaves the literal effect flexible depending on whether your edition uses exact target numbers (as in 10th edition) or chance to succeed (as in Horus Heresy).
There are, of course, exceptions. The Warhammer Vault explicitly states that "outdated missions and unit rules" are excluded from its archived books, so Games Workshop understands that an edition can impact the relevancy of expansion material. However, I find that even after an edition change, the spirit of an expansions remains in tact. Suppose a mission-specific rule is introduced in a game scenario, and it triggers during the Rally phase but your edition doesn't have a Rally phase. Try a game with the new rule at the start of each round. If that doesn't work, try a game with the rule just after Movement. It only takes a little experimentation to adapt.
As I've discovered from reading lore from the very first edition of Warhammer 40,000, the in-world history of 40k is fun to experience in all its many forms. There have been lore changes over the decades, but possibly fewer than you might imagine. And even with lore changes, the authors cleverly retain plausible deniability by blaming the Imperium for its faulty record-keeping. Space marines were originally written as humans taken from feral worlds, but today are explicitly post-human genetic experiments? Obviously the Administratum scribe writing the original report had no idea what he was talking about, or worse yet, may have been a Xenos spy spreading disinformation!
Well, generally the more information you provide to a consumer, the better. It's the consumer's right to understand what they're about to purchase, and to "trick" somebody into buying a book by conveniently omitting the edition number is anti-consumerist behaviour.
However, I don't think that's what happens within the Games Workshop product line. Generally, there are 2 factors influencing the purchase of game books:
When you're about to buy a book for rules information, you're probably looking for something specific you've been told you need to play the game, either by another book or a website or someone who knows the game better than you do. I don't believe it's likely that you'll stumble onto the wrong edition under these circumstances, because you're looking to buy a specific product to solve a specific requirement. When I was new to wargaming, I had a lot to learn about wargaming terminology, wargaming products, and on top of all that I had to learn to navigate Games Workshop's unique way of selling their products. Not once did I accidentally purchase a game product intended for a previous edition. I didn't have to do any special research, and I was always able to find the product I needed for the current edition. It's not any different to when I buy an expansion for Mansions of Madness. I don't have to verify that it's for the current edition of the game, it just is.
In the second scenario, the edition really doesn't matter. I've purchased old codices and old campaigns exclusively for content that exists external of any game rule. I've purchased content as far back as 1st edition, 2nd edition, 3rd edition, and 7th edition, and I've gotten every penny's worth.
Warhammer 40,000 gets new editions too often, for my tastes. Personally, I invest in a core game once, and then I buy expansions and gaming supplements so I can play exactly that game, for decades. For that reason, I appreciate that Games Workshop understands the value of separating mechanics from game play. Mechanics, I can learn as needed, and it's a pretty powerful feature that game play remains fun regardless of nuances of the latest mechanics.
All images in this post copyright Games Workshop.