Think of the Guardian of the First Threshold kind of like the TSA: once the time comes for the Hero to journey into the unknown, the Guardian of the First Threshold will be there to make sure she's not bring a bottle containing more than three ounces of shampoo.
Er—make that: The Guardian of the First Threshold will be there to make sure that she's prepared to step outside the normal realm. (Extra large bottles of shampoo, not to mention knives/swords/ninja stars are usually encouraged on Hero's Journeys.)
The hero's going to face all kinds of challenges on the road, but one of the biggest comes when she gets to the end of the known world, and steps into those parts that are dark and unexplored. The first guardian makes a great test to see if the hero is worthy of entry…even though the guardian can be a bit of a chicken and won't ever go out into the darkness him- or herself:
Such custodians bound the world in the four directions —also up and down—standing for the limits of the hero's present sphere, or life horizon. Beyond them is darkness, the unknown, and danger; just as beyond the parental watch is danger to the infant and beyond the protection of his society danger to the member of the tribe. (71.2)
And like the mentor, the guardian of the first threshold can sometimes turn out to be an ally, or at least put in place by an ally. But like everything else on the Hero's Journey, it's here to test the hero… and in the process show the hero what she's capable of.