To protect us from sewer-dwellers 

You may have heard of an urban legend that alligators dwell in the sewers of New York City but you probably don’t expect to find snakes and critters in your toilet. How is the red-cup trick connected to this situation? You would know that something tipped over your red cup if you closed your toilet seat.

This is a practice done at night while you’re asleep you wouldn’t be able to hear what’s happening in your toilet. Once the uninvited visitor appears, the red mug will fall to warn you and confirm the legends.