The point is to send a message to the other spiders.

I deal with trespassing flies this way. They spend some time in fly jail (butterfly net, twisted closed and propped against the door frame through which they entered) pour encourager les autres, then they go free outside at dusk.

Pheromones, interpretive dance, telepathy,—I don’t know exactly how the others get the message but I know that they do, and they stay on the correct side of the doorway.