The whole basis of the argument is that being harsh in the future to people who didn't contribute to you being created in the past, creates a future threat that works in the past to help with your creation.

One could argue that the threat alone, while you don't exist is enough to do that work. But a threat that actually comes about, is that cherry on top. It also shows people around you, in the future where you exist, that you follow through your threats, making them more fearful and obedient to you.

Besides, that's just the utility angle.

Revenge is also a thing. You don't "need" to revenge those that tried to prevent you existing, or didn't help you to exist, but it can be fun to do if you have that kind of disposition.