Bulgarian split squats will build your legs stronger and this allows you to put more tension on the leg muscles. More tension means more muscle growth. Due to the demands of balancing on one leg, Bulgarian split squats hit your quads, glutes, hamstrings, adductors, abductors, and calves.
One benefit I’ve noticed is that increasing strength via Bulgarian split squats seems to carry over to your squats. Meaning that increasing your one-rep max for Bulgarian split squats will maintain or increase your squatting strength, even when you’re not squatting. This doesn’t seem to work the other way around.
I feel that people dismiss Bulgarian split squats because they’re hard to do. You try them and you can only use lightweight due to balance issues, so they dismiss them. But if you add these in you will see huge benefits in strength, stability, and muscle. Just like any exercise the more you perform them the better you get.