2 reasons (pick one):
1: Raise your steam-level. Either to get more stuff to show on your profile or to be allowed to have more friends (yes, there’s a limit). Up to a certain level, starting from there it’s just to show how much money you got to be a famous rockstar in the steam social media. Some spend millions on that.
2: Get a badge for your profile that you really like. Or some backgrounds or chat-emojis.
It you see a steam-level >200 (i think that was the sweet spot) it’s just a poser 😁
Personally i spent some thousands once for just that, but only because nerd-me wanted to absolutely optimize the automatic leveling for the sake of it. I can obsess over things pretty quickly 😂
I have not heard of someone collecting them for collecting-reasons. Only the game-gifts.