The less obvious benefit of having the document ready.
Beyond any specific legal requirement, having an IDP alongside your license removes a layer of uncertainty from a trip — one less thing to wonder about at a rental counter or if you're ever stopped by local police. That peace of mind is a real, if less tangible, benefit of the document.
For travelers who'd rather not spend mental energy second-guessing whether their documentation is sufficient, having the permit ready in advance settles the question before the trip even begins.