The rear bushing on the stock e30 has a centered hole for the front control arm. The M3 bushing is offset, the hole is moved toward the center of the car about 1/2" to change the suspension geometry. The rubber is designed for greater durability too. It makes turn-in sharper and increases caster IIRC, to sharpen the handling. I'd figure it's best for track cars, but plenty of people on here upgrade to them, especially if they're adding an e36 steering rack.
They're required for a 5-lug conversion too, along with mid-90s M3 control arms.