From a thematic standpoint, “Outsiders” is supposed to represent how the faction lives more with the trees than the denizens and is looking to extend the forest into the clearings. From a mechanical standpoint, it’s mostly meant to prevent the faction from triggering the Alliance’s Outrage multiple times (imagine a Synpathy token in a target and original clearing; the potential for 4-6 cards per swarm is overpowering for the Alliance and too damaging for the citizens).
In practice, when you take the “Converge” action, the warriors should get moved from their clearing, down a path, and stop on the path right before the edge of the clearing. I’ll try to post a picture later. For that particular battle, they are considered to be “in” the clearing and count towards the max rolled hits (even though they don’t technically “enter”). When they “Disperse”, they return back down the paths they were sitting on to their original clearing.