Gullible folks who believe in old wive's tales and urban legends follow this practice.
Since plant wetting results in plant growth and thus larger plants, the above unsuccessfully transfer this logic to bed wetting.
These cheapskates note that single beds cost less than doubles, queens or kings, so they water them. Of course, this does indeed make the mattresses grow, until the beds become jaguars.
There have been no validated, reproducible or reputable studies lending credence to this theory, yet millions stain their mattress in this futile procedure.