Para EQs offer more tweakability than than graphic EQs.

The frequency of each band and also the Q (shape width) can be set.

There are different shapes you can use, one is called "Bell" because the shape looks like a bell, lets say you have hum at 50 (or 60) hz, with a para EQ Bell band you could set the frequency at 50hz and dial in a very narrow Q with negative gain to basically just cut out that frequency and remove the hum.

By widening the "Q" we make the Bell shape fatter so it affects a wider frequency range.

Using a Para EQ you can finely adjust your frequencies, they are extremely useful.

There are also other shapes, this is nice little intro: http://audio.tutsplus.com/tutorials/production/how-to-use-a-parametric-equalizer/

Cheers

Andy