Actually, like most sitcoms of that era, it was filmed in front of a live studio audience, it's not a canned laugh track. The reason it lasts just long enough to overlap audio is because the actors wait for the laughter to die down before continuing. The whole point of the laughter isn't to make it funnier to the viewer, it's so the actors can see how the jokes land and tweak their performance, it's constant feedback. It's done for the same reason comedy has an audience even when it is a filmed special.