Depends very much on the country. Many countries don't care, some like Japan will absolutely refuse to let you drive a car without one.

And it has to be the correct flavor of IDP too, meaning proper 1949 Geneva Convention on proper 1940s cardboard, none of that new-fangled 1969 Vienna Convention nonsense. Oh, your country has signed up to 1969 but not 1949? No car in Japan for you then.

Oddly enough, no car rental place has ever checked. Supposedly the police care, but I haven't seen much evidence of that either. Given they only cost $25, I keep one with me anyway.