Futurama has the only good grandfather paradox plotline/episode, and I'll die on that hill. In every other case its a sign of poor writing, as it steals agency from characters while stealing some or all suspense from the audience. It makes for tidy explanations and not-quite-deus-ex-machina plot points, but otherwise kills the story. Enterprise fell into this trap, when it didn't need to - it had everything going for it without that.