The UM are also responsible for over half of the SM Chapters existing in M41. Seriously. 60% of the current Chapters in the Imperium are UM Successors. They are the granddaddy First Founding Legion that no other, single Legion-Turned-Chapter can hold a candle to.
They are the "default" Space Marine. They don't have the baggage that any other published Chapter has, if you want to approach them as someone who might be new to the game but is somehow well-versed in the lore. The UM hasn't pissed off every other organization in in the Imperial body-politic (like the Space Wolves), they aren't tied to a specific Imperial faction (like the Grey Knights), they aren't stuck chasing down their own traitorous members from 10,000 years ago (like the Dark Angels), they aren't Vampires in Space (like the Blood Angels). They aren't crippled by a freak battlefield accident (like the Crimson Fists), and they aren't stuck in the middle of the Imperium defending a planet that doesn't see a whole lot of action (like the Imperial Fists), and they aren't a one-trick pony of a Chapter (like the White Scars).
They are versatile in their adherence to the Codex Astartes. They have so many off-shoots that you can easily create a home-brew Chapter and use the UM army lists as a guideline. Borrow three of their eleventy million SCs/ICs, slap a new name on them, and, bam, done.
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