Most dishwashers are designed to drain into the disposal, but they usually still have some kind of filter going to the sump before that line. Maybe you have some extraordinary model which truly has no sump filter, but otherwise you should still regularly examine and clean out your sump filter even if your dishwasher drains to the disposal.
The water in the dishwasher is being reused by the pump and sprayer while in cycle. The sump filter ensures no particles larger than the sprayer heads are being sucked up by the pump and pushed through to the sprayer jets, as tough food particles could damage the pump or get clogged into the sprayer heads.
