i normally dont fat during leg presses, but i one of the best workouts from leg presses ive gotten was when i had to fart...i knew if i farted id laugh and fuck up my set so i heled it in while i leg pressed and for some reason it beat th eshit out of me
quads give you the size, but hams give the 'complete' thigh
when you do any side-chest or tri poses, 3/4 poses, if you dont have hams, it'll look messed up
Here's a simple way to think about it. Quads four muscles. Hams two muscles. Quads are the larger portion of mass but big hams give that balanced look like d3 said.