Because they really work the top of your knees your lower quads I think. For me on the Squat which I want to master and pound out more weight . My lower knees are my week point. I can lift alot but cant go parellel to the floor becasue of knee pain and or lower quad weekness, I give out.
My idea is doing heavy hack squats concentrating getting up from the low position. Maybe for 2 weeks then jump back to squats. My form has gottne better on squats and I have upped the weight but getting low is really hurting my lift. thought 2 weeks of this should help. I find hack squat to be almost as difficult as the Squat. Mean while other forms of Leg work I am still getting good gains. Deadlift 405 twice "done on back day", Leg Press 1000 for 8 - 10 reps. Leg ext 300+ for 8 -10.

I hope yall agree I thought it was a good idea. Just to help my squat.
Now tell me how incorrect I am and what I should really dotuttut