Requirements vary more than most travelers expect.
Requirements differ significantly by destination: some countries require an IDP for any foreign driver, others only require one if your license isn't in the local language, and a few don't require one at all for short stays. Because the rules vary so much, checking your specific destination's requirement before travel is worth the few minutes it takes.
Even in destinations where it isn't strictly required, having an IDP on hand is a low-cost way to avoid any ambiguity with local authorities or rental staff.