Nothing. Having a toggle for "legitimate interest" is nonsense. The GDPR lists some exceptions to when you need to ask for permission, these are "legitimate interests". Things like remembering someones IP to keep track of bans is allowable without needing to ask for permission.
Of course advertising agencies promptly went to work trying to bend the language of GDPR so they can claim they are a legitimate interest and therefore exempt. It won't hold up in court.
The GDPR is surprisingly strict, and a LOT of the cookie popups you see in the wild are not at all compliant. To give an example: having your "accept" and "reject" buttons a different font size is explicitly not allowed.
Sliiightly more debatable, but you're not supposed to emphasize one over the other iirc. Go read the GDPR, for legalese it's surprisingly readable.