Monster disguises are mainly used by the villains of various foes of Mystery Inc. Most of the time these are used to scare intruders away from some sort of criminal endeavor such as a smuggling or counterfeiting. Other times they are used as a disguise while the villain tries to steal an object such as a rare jewel or valuable formula. Most people are fooled by the costume due to monster mask magic, which makes the disguise look very real. A few who have used these disguises have done so for noble reasons such as protecting an archeology site from tomb robbers. Mystery Inc. members Velma, Shaggy, and Scooby-Doo have been involved in such scares.
In the Supernatural crossover episode Scoobynatural, this was briefly touched upon with Velma, while hunting a real ghost, pointing out to Sam Winchester that it tends to be greedy real estate developers in disguise that are responsible at the end of each case. Sam protests that there are better real estate scams than disguising yourself as a monster. At the end of the case, the Winchesters and Castiel have the ghost pretend to be Cosgood Creeps in disguise to grant Mystery Inc. peace of mind with Dean commenting that his favorite part is always the gang unmasking the bad guy. The ghost's shapeshifting powers allow him to successfully fool Mystery Inc. into thinking that they really did catch a human bad guy in disguise.