why use a prime lens?

So you're thinking, "well, I'm buying a camera, and geez I don't need all those lenses. So I'll just get myself a combination lens and be done with it."
Well, I hate to disillusion you, but a combination zoom lens is not near as good as a prime lens. And here's the irony: a prime lens is cheaper to make because it's only one focal length, therefore the manufacturer will make an excellent lens because of that; where a combination is trying to get the best of both worlds, and we all know what happens there, right?
"what about optics?" you say, or "well, I don't have to get close to the subject".
In the words of Frank Capra: "if it wasn't good enough, you weren't close enough". Frank Capra photographed the storming of Normandy Beach. You see folks, that's the trick, get in close. It's all fine and dandy to use a combination lens when there is no other option, however, there are side affects.
Different focal lengths will distort the image in one way or another. This is why 135mm is the ideal focal length for doing portraits.
Ever see those shots of wildlife? The image is flat because there is no depth of field. That's because a lens with a long focal length was used, so it compresses the image.
Ever see shots, say sports, where out-of-focus background images appear to be fuzzy donut rings? That's the side affect of a mirror lens.
Ever wonder why 50mm is the standard lens? Because it's an exact 1:1 ratio of what the eye sees.
Sometimes I find myself lazy using a combination lens, but I force myself to use a prime lens often. This way I have to physically move in order to get close to the subject and crop the image.
For prime lenses, I use a 50mm on my Canon, and a 135mm on my Pentax. The optics are tact sharp, and the lenses are fast. But each camera has an 80-200mm lens.
Sometimes when a subject is far away, and I mean really far away, I use a Sigma 600mm mirror lens on my Pentax. But of course I know the side affects of using that lens :)
So you see, it all comes down to knowing your equipment and understanding what different focal lengths do.
So go ahead, experiment with a prime lens. You will really see a difference :)