I live with Indonesian parents and they say that when we do something improper like speak rudely, or if someone repeatedly gets something wrong. It can also be when someone hears a rumor about you and you're trying to defend yourself. There might be other situations but this is what i can remember

I think 'betul' is 'correct' and 'benar' means 'true'

They're synonymous. You can use them interchangeable.