As I understand it, the offset ones change the camber(?) settings. It's supposed to give sharper handling. It also can affect tire wear. If you use them, you probably need to check your tires more often.
1995 325I, tinted windows, K&N, debaffled air box, BMW style #104 wheels, Nighthawk low beams, smoked side markers, smoked corners, smoked taillights, M3 style heated mirrors, various interior bits, Bosal cat-back