I can think immediately of some situations where it could be useful.
Very recently I was working with a group of divers doing some work clearing out collapsed areas in a cave. That work raised a lot of silt, and between efforts we needed to let the cave "rest" to clear out. At the end of the day, the idea was raised of going into a section that was especially problematic and intentionally digging out some silty areas, creating a super silt out. We decided that if the diver doing it went to the deepest level first and then moved to the exit in a deliberate fashion, pulling out silt as he went, he should have no trouble getting out. It was, however, a one man job. I was that one man. I made a time table for doing the work and getting out. If I violated that time table, someone would come in and look for meme. (Everything went as planned.) If I had had such a device, I would have used it for an extra measure of safety.
I can think of other cases where buddies deliberately separate because of situations like that, with no clear way to signal trouble. If you read The Last Dive or Shadow Divers, you know about the Chris and Chrissy Rouse case, for example.