His jB is good when you know how to space it well and not telegraph it. It has a large variability in frame advantage. If you use it too close, then it’ll be fairly negative on block. If you use it from further away or closer to the ground, you can actually be positive. With that said, here are some scenarios that are useful:
You can use his jBB for a round-ending chip setup. If the opponent has 1 life, you can send out ghost and then follow up with jBB. If they block ghost, they will end up blocking your jBB as well. All three hits will cause 1 full point of damage on block. This is usually used after they have been knocked down.
You can use his jBB as a combo ender from his crossup ghost regrab combo. The full combo is crossup jA, A, C, jBB. It does 4 damage and knockdown.
Against Rook, if you see that he has tried to use an armor move, you can immediately jBB for a free 2 damage and knockdown.
At specific spacings, you can crossup with jB. It’s a bit character dependent, and it’s a nice mixup.
Because of the hang time you get with jB sometimes you can sail right over a projectile and hit the opponent.
Finally, if you use jB safely, you can also hit confirm into a second point of damage.
Those are some of the situations his jB can be used.