Why might members of one political campaign want to spy or steal information from another campaign?

So they can foresee something they might say or do and prepare to counteract a statement they might make or do something they planned to do first. A good (and funny) example is in an episode of Parks & Recreation where Leslie Knope announces that she's going to put a wheelchair ramp by every set of stairs in Pawnee but then the other political party campaign announces that they're going to put wheelchair elevators next to every set of stairs in the town.