Think of the hero as the Paul Revere of characters. He may not be the one making the world's biggest batch of iced tea in Boston Harbor, or even the guy strapping on his buckled shoes to go kick some redcoat butt…but he's definitely the one screaming "The British are coming! The British are coming!"
Because somebody needs to wake the hero up and start him out on his Hero's Journey.
You need an adventure for the hero to go on, and sometimes that has to come at the call of someone else. Enter the herald: the one bringing the bad news that a hero is required right the heck now.
As Campbell puts it:
The herald or announcer of the adventure, therefore, is often dark, loathly, or terrifying, judged evil by the world; yet if one could follow, the way would be opened through the walls of day into the dark where the jewels glow. (48.1)
That can apply to heralds who ultimately turn out to be good, Gandalf -style. Or they might turn out to be faceless stooges who are absolutely no help whatsoever (like these twerps knocking on the door of our favorite archaeologist.)
But once they show up, nothing's ever going to be the same again.
And once they show up, their job's pretty much done. Some of them may morph in mentors (like Gandalf), but others may just fade into the background, having deposited a big wet mess for the hero to find a way to deal with.