Planes don't tank more than needed for any flight.

That is just an objectivly wrong statement.

Fueling doesn't happend instandly and the whole process (inluding new fuel calculations, calling the Fuel-Crew, driving up and attaching/detatching the tanker, signing off paperwork, etc) can take up to an hour, without a single drop of fuel being filled into the tanks.

Planes often fly multiple short hops, well below they maximum possible range (e.g. between the Hawaiian Islands). If the pilots calculate, that they can stay below there landing weight, then they might opt to take fuel for multiple flights. Burning a few hundred kilos more fuel costs less than having a full crew twiddling there thumbs & letting ground personal run around for an hour.