Guardians are always necessary in this day and age, especially as the line between reality and fantasy becomes thinner and thinner. The Night Watch is an affliate of the Overworld's, contracted specifically to keep an eye on things in the mundane world and stop any nightmares or evildoers from gaining a foothold there.
Their affliation is loose, however, and they are sometimes seen as a group of mercenaries, hiring out their services to the royalty of the Overworld. They don't work for free, paid protectors of the current status quo. Night Watchmen come in all sizes and shapes and flavors. Dream Constructs of a more combat vent often find this line of work to be their forte, allowing them a kind of existence they wouldn't normally have, outside of the Otherworld. Lucid Dreamers may also be hired in to help keep an eye on things.
The Night Watch is a very vigilant group, keeping extensive files on those who have been declared criminals in the Overworld. Some day they've gone too far these days, rumors of unwarranted arrests and unlawful detentions surfacing lately.
(I'm sure I'll think of more to add to this.)