Prevent himself from refusing to [publicly] leave office in peace, and prevent his enemies, our enemies, from instigating mass violence and unrest in response. The media predicted Trump hunkering down in the white house and causing violent civil war. By leaving in peace, while still claiming the election was stolen, it keeps his supporters motivated but not violent, keeps his enemies for having an excuse to cause violence, and all the while let real work be done behind the scenes by whoever was actually wielding the powers of POTUS