A lot of times it is a sorta "good or evil you're still screwed if you don't." Example: skyrim. Alduin is gonna eat the world. You're part of said world. Kill him or be eaten along with everyone else.
I had hopes for nukaworld but it fell pretty flat. The way the built it up I was expecting something similar to an alternate evil ending for the main story or something.
I'm just taking out my competition.