Limited slip differential. If one rear wheel starts to spin, a 3-series (non M) will just lose usable power because all the engine torque will go towards that spinning wheel. Either that or the electronic nannies will kick in and kill engine power/apply brakes. A car with an LSD will transfer some of that torque from the spinning wheel to the non-spinning wheel.
Useful in corners.
Always laugh when you can. It is cheap medicine