Planes dump fuel to get below maximum landing weights. This typically happens when planes encounter an issue right after takeoff, and need to return to the airport.

There are two kinds of emergency landing - those that happen in a hurry, and those that happen slowly.

If it happens in a hurry there's no time to vent fuel. If it happens slowly fuel can be vented, but typically just enough to get below max landing weight. It's not like they "empty the tanks".