So, when should you actually use feature flags?

Hey everyone, I've been thinking about feature flags a lot lately. We've used them for rollouts and quick turn-offs, but I'm curious about other situations. When do you guys find yourselves using feature flags? Is it mostly for A/B testing, or do you use them for things like moving away from old features, or even for dark launches? Any specific ways you use them, or any problems you've learned about the hard way?