Zack's change would have been too big without those videos in my opinion. The idea is probably that while Sophie has forgotten about Felix, Zack remains a sadistic person. So saying that Zack gains nothing from it is wrong. We don't have his PoV, so we literally don't know, but if we had it, it would be clear that he enjoys that. Can you imagine him sending the video thinking "yeah this is totally useless"
. Maybe he gets hornier knowing Felix watches that stuff I don't know, but he gains something from it. Also, it plays on the fact that Zack knows that Felix doesn't have the balls/still loves Sophie to show those videos around, and that's in fact what happens, because he still thinks of her. So the risk is basically zero.