Sometimes my to do list is the only record of a given task I need to complete (e.g. someone says something verbally, I add it to the list, and there's no text or email regarding it). If it's getting deleted at the end of the day no matter what it's going to force me to complete those tasks lest I forget about them entirely.

But more importantly, why are you asking what the benefit is of a self-described "dumb thing"?