Born out of the militia assembled by Hancock to overthrow former mayor Vic and his thugs, the Neighborhood Watch serves as the closest thing to a standing army and police force that Goodneighbor has. Defending their town from without and within, so long as visitors and citizens alike behave themselves (more or less) the Watch will take no issue with them.
↑Fallout 4 Vault Dweller's Survival Guide Collector's Edition p.38: "NEIGHBORHOOD WATCH The Neighborhood Watch is responsible for keeping order in Goodneighbor. Acts of theft or violence are answered with a hail of bullets, but visitors who stay out of trouble have nothing to fear from these local guards. Wearing only patched three-piece suits and worn fedoras, members of the Neighborhood Watch aren’t particularly well protected. The group does include both humans and Ghouls, however, so some members are immune to radiation damage. Most of these guards are surprisingly friendly, but they’re always quick to deal with misbehaving visitors." (Fallout 4 Vault Dweller's Survival Guide Groups, Gangs, and Factions)