Those things fascinate me!
Trouble is, you could spend so much time fiddling with it- (especially that particular unit which I would guess was probably meant to be used with a contemporary PA system), you might not get much actual playing done!
I once had an amp with a built-in 5 band graphic EQ- I used it a lot to modify my guitar sounds- very useful, as I was in a band at that time which played a very large range of material- literally every style from trad jazz era standards from 20's/30's to the then current Top 20(late '70's/early '80's)!
I swear this is true - around that time, I was once playing a friend's Gibson Les Paul Custom with that amp, and I could make it sound like Mark Knopfler's Strat on Sultans of Swing....
These days, I'm not bothered so much about getting exact tones to match records etc but to me a graphic EQ is a very useful tool to do this, and much easier and cheaper than endlessly swapping pickups in and out of the guitar, chasing the chimera of the perfect tone...