I just finished reading Sakurai's treatment of feynman's path integral, and I'm left feeling really stupid. So the integral gives the propagator, which represents a transition amplitude. I'm left wondering what we use that for. Perhaps I'll understand when I start working some problems, or perhaps after I read the derivation a few more times. But, just to cover my bases, can anybody recommend a different source of the path integral derivation (which, according to sakurai really isn't a derivation)?