Use this phrase when someone is asking you to give an opinion about something that you don't really want to answer, but someone is making you answer.
You can use this phrase with different words that mean "say". For example:
If I had to guess, I'd say that there's probably 2 more weeks' worth of work left to do.
If I had to choose one, I'd say the Civic is probably a slightly better value for the money.
When you use this phrase, you sound like you're being careful.