The rat catcher was a surprisingly well-respected fellow, particularly after popular opinion decided that rats played a role in human illness. Historically, this opinion shift is related to people first thinking cats were witches’ familiar spirits, deciding cats were therefore unlucky, and killing cats in order to be safe from witches. The reduced cat population led to an explosion in the rat population, after which rats were considered unlucky. As the name implies, the rat-catcher captured and killed rodents, protecting personal dwellings and critically important food stores.
In a D&D game, a rat catcher probably has a great deal of information heroes might want. It is very likely that he has been inside of nearly every building in his town of employment, and people tend to ignore him while he works. Need to help a companion escape from jail? The rat catcher goes in there twice a week, and knows about guard shifts and passwords. Want to know where the Lady Alandra keeps her correspondence? The rat catcher probably knows. Unscrupulous rat catchers may be found in the employ of roguish organizations, selling their knowledge to the highest bidder; even honest ones may be motivated or duped into sharing what they know by compelling argument or careful questioning.