Due to inflexibility, injury or other problems, not everybody is able to do the back squat with correct form. Or you can throw in other exercises just for more variations in your training. Switching to a new exercise can get out you from boredom, or it can help you break through a plateau.
Every exercise trains your muscles and nervous system in a - more or less - different way.
You have other options in squatting.
Front squat:
Barbell Front Squat (Exrx)
Front Squat (VerticalJumping)
How to Front Squat with proper technique (StrongLifts)
Box squat:
Barbell Box Squat (VerticalJumping)
How to Squat - Box Squats (DieselCrew)
How to perform Box Squats (StrongLifts)
