Why "prime?"

I'm still a newbie. I bought a Canon T2i about 6 months ago &, and have been shooting solely with the two"kit" lenses that came with the camera package -- an 18-55 and 55-250.

Tonight, I finally ordered a 3rd lens.......a Canon 50mm f1.8 . I figured I didn't have much to lose since it's only $120 and it sounds like I can take some nice portrait shots & low-light shots with this lens. I understand that it's built cheaply, but I don't have $$ in my budget right now for really good lenses.

Anyway, I'm curious why "prime"/ fixed focal length lenses are so popular? Reading through reviews & these forums, it seems like a lot of people rave about using "prime" lenses rather than zoom / adjustable, and I'm curious about the difference in image quality, etc?

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Michael
http://www.flickr.com/photos/22298045@N05/