A high roll rate allows you to get "out of plane" (no, not bail )with the enemy plane behind you.
If he aligns he wings with yours then he is "in plane" with your line of flight. All that's required then is to pull a little lead (deflection) and boom! Staying out of plane with the guy behind you spoils his shot somewhat.
So, that's a defensive move.
Now the opposite holds true for the attacking plane. If you're flying a plane that has a better roll rate you will find it much easier to roll into plane with the guy you're following and no matter how he rolls you can easily remain saddled up for the shot.
Hope this helps.
Ren