There are several reasons to run swaybars. One is of course to prevent body roll. Body roll changes camber, reducing grip.
Note also that a soft bar give more traction than a stiff bar. Does the car push? Stiffen the rear bar or soften the front. Is the car "tail happy?" Stiffen the front or soften the rear. It is a way to tune the way the car handles for driver preference. (I've ran a Mugen MBX5 Prospec with NO front bar... it was the only time the car turned satisfactorily... but then it had other problems.)
With no swaybars, the car is more apt to bottom out one side of the suspension over bumps, and "upset" the car. The swaybars help with stability in rough sections.