Now that their mission is complete, Frisk's determination is forcing them to collect all the other scattered souls in order to become full again and delete itself so as to prevent any other resets from ruining the happy ending.
Frisk and the others were a random anomaly of nature meant to free the monsters.
If not the video proper, then the last half of the video showcasing the various Gasters used is definitely worth a watch. The project spans A Us, interpretations, and styles, and the sheer quantity of designs for the enigmatic W.D.