When do you need a hood?

How do you decide when you need to put a hood on a lens? I've read that a hood is supposed to prevent flare, but what are likely conditions for flare to appear? What does it depend on - the focal length, the angle between you subject and the light, filters used, anything else?
I even made a dozen or so test shots, and a hood seems to make no difference. Considering that it's bulky and very inconvenient, I'd rather not use it unless it makes my pictures better.
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