zoom telephoto vs telephoto

I am a complete newbie when it comes to slrs and I have a question. What is the advantage of owning a 300mm telephoto, if you could have a zoom telephoto that covers say 75mm-300mm Any feedback would be fantastic....
Tim

The following applies to pretty much all primes and is strictly on a "typical" basis:

You get much better optics. Sharper images mainly.

They are almost always faster, although there are exceptions...ie there are some f/2.8 tele zooms, for the most part they are all f/4+ where as the primes are almost always f/2.8 or f/4. Some are f/5.6 i think but those are 500mm+ which is pretty fast for that large of a lens.