Are they just the usual achievement-ish stuff or do they serve some deeper purpose?
Steam Badges only serve two purposes:
A) Add experience points to your profile
B) Add bragging rights
Experience points are required to:
A) Level up (duh)
B) Gain more slots on Friend List
C) Gain Profile Showcases
D) Have a chance of getting Steam Coupons for games
Without Steam badges, you won't be able to have additional room in your friends' list, showcase anything notable on your profile, or get Coupons towards Steam games.
According to this article and my own experience, they currently serve no purpose apart from bragging rights.
This doesn't mean that they couldn't use them later to reward people using steam a lot. (Like giving early beta access or other rewards.)