Fully independent rear suspensions allow for much better control and handling, but are bulky and expensive. One thing they don't deal very well with however are changes in supported load. The weight on the front suspension changes little compared to the rear when passengers, fuel, trailers, and cargo are added or removed. Hence, the popularity of torsion beam rear suspensions.
Standard vehicles are tuned for "safety", that is when approaching cornering limits, they will under-steer (plow straight) rather than over-steer and potentially spin. Without massive tech, most any cars needs anti-roll control via (anti-)roll bars. Adding a bar to the front resists roll and also creates understeer. With enough front stiffness for roll control on most FWD cars gives too much understeer and adding a rear bar balances that out. The stiffer the rear bar, the more "lively" handling gets, that is, it tends toward oversteer. OEMs want light rear bars. Aftermarket lets you go heavier to get a sportier handler.
