It depends on the legal relationship between your home country and the destination, so this is a check-per-country item, but the framework is simple. An International Driving Permit isn't a license at all; it's a standardized translation booklet of your existing license, created under international road traffic conventions, and it's only valid alongside your original license.
Here's the practical reality that decides it for most travelers: even where enforcement is loose, rental companies and insurers can require an IDP regardless of the law, and an insurer can deny a claim after an accident if you weren't licensed to local standards.
Two cautions: IDPs generally must be issued in your home country before you travel, not after arrival, and online sellers offering international licenses independent of your real license are scams.