My undertanding is that it makes it easier for a rescuer to cut the bridge if they need to quickly release a disabled climber from their tie-in. The knot provides a "stop" for the knife blade on the bridge. This cut location leaves the rest of the victims rope system intact.

The rescuer in the tree would be tied in to a separate system and would secure the victim with a pick-off strap before cutting the bridge. Cutting the victim's tie-in is used only if they can't be lowered on their own system.
-moss