I've been doing single leg box jumps every now and then for the past couple of years. I love doing these over normal box jumps depending on the goal of the workout.
Single leg box jumps are great if you want to improve unilateral explosiveness. Running is somewhat of a "bound" from one leg to another. Adding the seated aspect creates more of a challenge. It makes you rely on your fast twitch muscles (type II) to swing your leg forward and use the elastic energy from the seated to the single leg position to propel yourself vertically.
I wouldn't recommend these to beginners or younger athletes as single leg jump requires a lot of coordination, strength and proper equipment. It's potentially dangerous if you aren't careful or dont know how to do it. That's why it's essential to understand proper leg cycling when running as it translates to other exercises as well.