The Barbell squat is the premier leg exercise. It is perfect for building big strong legs. You squat down and stand back up. Simple. Best way to train legs period. But on the other hand many squatting records have been set from box squatting stance. So why?
Box squats allow a more controlled eccentric phase to the movement most average lifters usually don't control the downward phase to the squat. But with the box or bench between you legs you have no option but to slow things down.
You are 40% stronger on your way down on a squat. So you can technically sit down with more than you can stand up with. If you want to improve on strength then I would suggest adding some box squats into your routine and reap the benefits and hit some new PB's