I've talked directly to members of several police departments about why they want unmanned aircraft. In Brevard county FL, for example, they want infrared cameras to help find patients with Alzheimer's disease that have wandered away from home. Several police departments want a cheap way to take aerial photographs of accident scenes so they can move wreckage and debris off the roads faster and keep traffic flowing. In general, it's for surprisingly mundane tasks. I happen to live in Colorado, and I'd love to use our products to help with forest fires, but the FAA won't let us.
Of course, any tool can be abused. It seems like that's the primary fear that people have with this new technology.