The only way they could be seen as an important part of the game is if you play games to play dressup.
You really shouldn't be but I feel you'd be surprised how many people, especially MMORPG players, care about this. I personally play GW2 a lot and since there's no gear treadmill unlocking cosmetics is one of the main goals you can actually do there, to show off your accomplishments or hard work. Ofc it's not the only thing keeping players engaged but it is a major part of it.
Besides, if it didn't matter to players then it wouldn't be monetized as much in the first place. So whenever I see this argument "well its just cosmetics so it doesnt matter:", I get kinda frustrated because obviously it matters, obviously it works which is why it's still a thing to monetize so much.