One theory as to why cats toy with their prey is they are tiring out and confusing it so they (the cats) reduce their chances of getting injured when they close for the kill.
My own hypothesis is that cats catch prey and play with them for a while as a way of getting more training per kill. They probably find some enjoyment in it, which is likely evolved as an incentive to practice. Cats are certainly well known for their playfulness, which tends to be stronger at a young age.
I suppose both theories might be true at different times, depending on the context or the prey. I guess mice aren't particularly capable of doing any real harm to a cat.