When you have a villager in boxes and no one comes to your town to adopt it, it lingers in the memory of the game and when you interact with other people via wifi or you streetpass someone, there is a possibility it might move in to that person's town uninvited, and it is not very pleasant to turn on your game to find a plot in the middle of your paths. When you have ten villagers though, there is no risk of it! So it is a courtesy to let people know that you have a villager in your void