The exercise ball leg curl is an exercise targeting the hamstrings. In particular, it targets the knee flexion function of the hamstrings, as opposed to deadlift variations, which target the other hamstring function, hip flexion. Many lifters opt for this exercise when they don't have access to a lying leg curl machine, but it is worth considering even if you already do machine curls.
Builds muscle and strength in the hamstrings
Glutes and core also need to work to keep this move stable