Like do they have a use like fossils or do you just sell them for money?
The only "useful" thing you can do with a Rare Bone is use it in conjunction with Fling for a 100 BP move. Fling is generally pretty gimmicky, so I wouldn't recommend it. Outside of battle, there are no games in which the Rare Bone has a practical non-monetary application.
Rare Bone (Bulbapedia)
Rare Bones have no use in-game except for being sold to Pokémarts for §5,000 or used as ammo for the move Fling. However, they used to count towards a player's Fossil Count in Pokémon Diamond and Pearl.