As a replacement to often cryptic and undecipherable clipping groups . A way to simplify layer stack and ability to get a dynamic live masks from a separate stack of layers . Those layers could be put somewhere in the bottom and just marked as a certain layer comp.
It would be working same as linked smart object. The only difference is that the subject would be linked to the same documents group of layers and thus working live. Without necessity to open and edit outside files to make changes.