There's a really good explanation in this thread:

http://www.tdpri.com/forum/stomp-box/121702-graphic-eq-parametric-eq-pedals.html

In my (albeit limited) experience, a parametric eq is good to boost/manipulate frequencies to alter gain for fairly dirty sounds. Certain metal guys in the 80s used the furman PQ4 as the foundation for their dirty sound. Guys like John Albani who played with Lee Aaron, and the guys in Styper (not 100% certain that's what's used, but the rhythm guitars on Soldiers under command have that PQ4 kind of snarl) come to mind.

Cheers,

Ron