Well, i'll be blunt here then: The astartes are the pillar which holds the setting together. Now, i'm not about to dispute the importance or role any faction plays in this universe. Even those who have been overlooked or lack writers who seem to know how to properly handle them like the necrons, tau and tyranids all have their own benefits. Yet despite all of this, the astartes are a faction whose actions, conflicts and nature has helped shape the very setting itself. They're present at the start of the current age, they are intrinsically linked to both the major factions of deities fighting for control over the universe, and they serve as the face for what is commonly presented as both the ultimate good and evil of the setting. That might be simplifying things to be sure, but we can all agree that Chaos will traditionally be depicted as the invader or antagonist, even when the story is from their side.

Some might argue that the eldar would count towards such a role in shaping the universe, having sparked and been involved in several ancient events which led the galaxy to its final state. While this is certainly undeniably true, the key difference stems from several factors. The foremost among these is how the Horus Heresy itself is used as a starting point for the main timeline in many regards, understandably so given how the Imperium comes to cover most of the galaxy. While the Fall and the War in Heaven are excellent tales unto themselves, the Heresy is just old enough to shape the galaxy but just new enough to fully explore. With half attraction to the eldar stemming from the veil of mystery and mysticism behind them, they become a race harder to fully explore in the same regards. It also allowed for a much more definitive link between the varied factions on hand, as each and every one of them on both sides have some interconnecting history in some way, from their successors to the First Founding chapters by way of their origins and primarchs.

This point extends far beyond just in-universe meaning however and unto the actual writing itself. Following the revamp the Third Edition underwent, the Index Astartes was used as a basis for the new universe. It established the new ideas, laid the seeds for the identity of 40K people enjoy today and broke away from some of the inherent self-parody the Second Edition focused upon above all else. They were, to be frank, the trigger allowing 40K as a whole to stop being Adam West's era and focus more upon being Kevin Conroy's universe. Many followed in their wake of course, from the introduction of the C'tan and Tau Empire to some deeper elements of eldar mythology, but it was this basis they laid which made that possible.

A further key factor to consider atop of all this is the fact that the astartes are - and remain - the faction with the most lore surrounding them. While this has led to accusations of favouritism from Games Workshop, quite accurate ones in many regards, it's still undeniable that they have some of the most detailed histories and ideas behind them. In a time where Alaitoc is lucky to get more than a paragraph in its codex and we know little to nothing of the inner workings of the Dal'Yth Sept or Kabal of the Shattered Hand, we know far more about many chapters as a whole. The First Founding ones have tomes upon tomes of lore based upon them, while their successors such as the Black Templars and Flesh Tearers have become a recognisable name within the fandom on par with their progenitors. Even the likes of more obscure factions like the Mantis Warriors have entire books about them, allowing for a wealth of information to exist across a very broad spectrum. When you pick out a space marine chapter, as a rule you're going to get far more information about their identity and culture than you will with almost any other army.

Perhaps what makes them stand out the most though is one key factor: Their flexability. Above all else, with the astartes you can opt to shape and transform them into almost anything within reason, taking inspiration from feudal and far future eras alike. While this might be true of certain armies given how they recruit others, it takes on an entirely new level once you consider how it might affect a chapter. For example, the Imperial Guard might be able to field legions of space vikings who storm into battle armed with axes and laser weapons. With the astartes, you can field hundreds of werewolf space vikings, with sagas stretching back millennia, relics worthy of any legend being wielded among many of their number and even their psychic powers altering to fit their origins. If there's an idea - again within reason - there's a good chance a chapter can be written to fit it, from Arabic guerrilla warriors who enter bullet time to eastern European Gregorian warlords who venerate the dead, wear their skeletons and call their ancestors into battle with them.

Are there problems with some of these details? Plenty and i'd personally always argue that similar treatment needs to be given to many other armies, starting with the Imperial Guard and Craftworld Eldar. Despite that though, there's no denying the strength of this force and how many factors can easily be seen as turning them into the iconic warriors who serve as Games Workshop's figurehead in almost all matters.