for three reasons perform this exercise on a bench:
1_ for the safety of the shoulder: the bigger the ball is, the more shoulder flexion it requires which in this unstable position is risky.
2_ to get a better contraction of the abdominal muscles and limit hip flexors: in order to contract the abdominal area muscles, you need a good spine flexion. when the shoulder is flexed too much, it is hard to flex the spine optimally.
3_ for a clear presentation in the video also in case you wanted to go through the full range of motion, you can let your hip dip in(hip extension) at the end to get a good stretch of the front core muscles but make sure you have it under control and do not exaggerate it.