What are these for?

for the short part of the safety belt - I think ?
when you move the seat back- or forwards that this short part will keep in place

 

The bra of your girl friend ?

 

Actually, Turbo-joe is correct. That elastic part is a “keeper” for the seat belt buckle. As you can see from your photo, there is a snap on that leather covered elastic part and that snaps to a stud on the outer portion of the inside seat bolster. I personally don’t think it works very well in maintaining position of the seatbelt buckle, but that is what it is supposed to do.