This one comes from Client feedback, in which it was flagged that there was a desire to be able to Expand All or Collapse all levels of a hierarchy in one click.

Photoshop achieves this behaviour via Alt(Hold)+Click on the Parent Element's Expand/Collapse Arrow.
(See attached video for a demonstration of this in action)

The way that it works, is that if a structure exists in which:

Group_1
....Element_1
....Group_2
............Element_2
............Element_3
....Group_3
............Element_4
............Element_5
............Group_4
................Element_6
................Element_7

If [Alt] is held when the Group_1 Expand/Collapse arrow is clicked, it will expand all of Group_1, all of Group_2, all of Group_3, and all of Group_4.
The reverse operation occurs if any number of Groups are already expanded, and holding [Alt] whilst Collapsing, will collapse all Groups, and all SubGroups.