Side split squats are probably one of the most overlooked exercises on the gym floor and without good reason.
Pretty much all squats work in the sagittal plane. Split squats are the only ones that [I know of that] work in the frontal, which means that it has the potential to improve mobility and unlock new muscle fibers.
They work wonders for building strength in the gluteus muscles. As long as you are getting your legs down to a 90° angle you will be hitting gluteus maximus – the largest in the group. Squatting with all of your weight on one leg, with knees outside of your hip width, will bring gluteus medius into the movement. Gluteus medius is meant to stabilise the hips and knees.
Side split squats are also great for correcting hip imbalances that may occur when performing normal squats, which is something that is very common. A few weeks on these babies and problem solved (in most cases).
More than enough reasons to work side split squats into your leg routines!