Mainly used for live sound, where you use them to correct the room somewhat, and to pull out the frequencies where feedback occurs- they are the kinda thing you set and then never really look at again. They are dual units, so that you've got one for the left and right channel. If you're looking for a nice EQ for guitar, try and find a parametric EQ; one with a couple of bands, and perhaps a shelf... much more flexible :)
