The entire purpose of using track spikes is simply to increase the traction you have on the track surface.
Imagine you tried to run on ice with your running shoes. You're not going to have any traction and you're not going to get very far. Actually you'll probably slip. If you were to use track spikes, the spikes would dig into the ice and you would gain significantly more traction.
It is the same principal when running on the track surface. The spikes catch the surface and give you more traction. Thus there is less "slipping" of your shoes on the track surface.
All in all, it is basically used for performance optimization. I'm not sure if there are any studies done on whether or not they actually increase performance, but I personally would suspect they do.