[greencreeper]

It's what's known as a "design fault". My parents had the same problem. It's supposed to collect water from the fridge and evaporate it due to heat from the compressor, as bopdude says. But the water collects faster than it evaporates, so it builds up. The water goes manky and collects dead wildlife. All you can do is buy a new fridge without the feature, or empty it regularly.