As I understand it, rollers are there to help the horse relax, and to prevent certain problems. When the horse pushes with its tongue the rollers will spin. This is supposed to help prevent the horse leaning on the bit or getting its tongue over the bit.
That said I have never used a bit with rollers, all of my horses have been in eggbutt snaffles. If i'm wrong i'm sure i'll be corrected.
Hope this helps