When the informant was a little boy, his parents would have a babysitter come over to take care of him and his sister.  After the informant and his sister finished dinner, they would bring their plates to the sink to wash them.  The baybsitter would say, as they approached the sink: “A monster lives in the sink, below the drain.  Be careful!”  The informant and his sister would very carefully wash their dishes, being sure to not let their hands go near the drain.

Years later, the informant now realizes the babysitter was simply protecting him and his sister from sticking their hands into the garbage disposal.  The informant thinks this was a good way to keep him and his sister out of danger.

I would add that this superstition also tapped into many children’s fear of darkness, making it an even more effective “safety-superstition.”  The scary noise the garbage disposal makes when run is the icing on the cake.

So that they wouldn’t put their hands down there

Good way to keep out of danger—potential harm