>>14215759
Well, think of it like this.
You know how the Guard had, like, doctrines? And you could combine those doctrines to make your own unit by having closed-rank guys or camouflage cloaks or stormtroopers or whatever, based on what you wanted the theme of your unit to be, but at the same time you could also use pre-made IG regiments that were assembled by the same process?
Space Marines should be more like that. Different traits and concepts to choose from, and then some of that reflects on their heroes - tactical leader types versus brutish "in the thick of things" commanders. This would allow you to add some customization and character to your units.
Obviously counts-as can do something similar, but in that case it's more about which pre-existing character you choose to copy, rather than building and customizing your own. I'm not trying to say people shouldn't use pre-existing units, but I am trying to explain why someone might prefer to use custom chapters / squads / characters.