I think it's automation and efficiency. Just like LE used to follow people around all the time watching where they went and such. Thing about that though is it's time consuming and if you were watching for it, you could start to detect it. Now they can attach GPS to your car and now it will just call in where it is at all times. It makes decision "Should we pay someone to following around potential bad guy X?" to extremely easy. "X might be bad guy but hell, GPS is only 2000 dollars and 50 bucks a month we pay to Verizon for cellular data plan, go ahead and put it on his car. We will figure out soon enough"
Same thing with License plate readers. While someone could sit somewhere and write them all down, that person could have messed up, it's expensive, they may not get them all. Now, point cameras everywhere, much cheaper, much more efficient and database storage is cheap, keep all the data forever.