I have never been asked for an international driving permit, but I take one when I'm planning to drive in a country for which my local motorists' association say I ought to have one. I figure that, at the price, it's worth playing it safe, and if one is required and I don't have one, I really don't have a leg to stand on if I'm refused a car rental or the police want to see it.

As noted above, it is essentially a translation of your own driving licence into five or six languages. It has no validity without the driving licence proper, so you need to take that with you as well.