you spar with one pair and you use the other on the bags... :P

Just the way the weight is distributed, theres more padding on the knuckes and stuff etc.... all sparring gloves have a linked thumb and most new ones have a white target area... you'lll end up knocking the stuffing out of your sparring gloves if you used them on the bag which would mean that your where using practicly unpadded gloves to spar.