The purpose of strut bars is to reduce flex that these towers experience during hard cornering. When taking a turn a car's strut tower normally flex, resulting body flex and losing some traction. As a strut tower bar is designed to keep your car's strut tower from flexing, they distribute the pressure applied to one strut tower when taking a turn to both towers instead of just one. This keeps the wheels in position. Keeping the wheels in position helps keep the tires in the desired position on the road, and this will help to improve traction on the turns.
Strut bars are a must for any upgrade to a wider tire&wheel combination. Addition of suspension upgrades such as sports springs, shocks and low-profile tires adds additional stress to the chassis which result in chassis flex. Strut bars not only reinforce the sub frame and improve chassis stiffness but also make steering quicker and more responsive.