Lots of things!
A shoulder to cry on, a companion when you need it (and sometimes when you don't), unconditional love...
Oh, and help taking care of new tattoos and piercings.
There is NO way I could have taken care of my back tattoo by myself. I knew that if I wanted something there, it would have to be when I lived with Baby A or someone similarly awesome and close to me. Baby A it was.
Aaaand I recently got a nose ring. I didn't have enough hands today when I was messing with it (cleaning it, moving it, making sure the hole in my nose is A-OK), so Baby A was called in as back-up.
The things we do for each other.