Essentially, a way to calculate how much of a Region3 has been filled, as a percentage of the volume of the Region3.
It would be cool for telling people how much of something is left, or how safe something is (e.g. a mine: you would be able to detect how much blocks are in the mine, and then depending on various factors cause cave-ins, etc.)
I’m not sure how complicated it would be, but I can definitely see it being useful.
You could honestly do this yourself if you really wanted to. Just break your region down into smaller and smaller pieces, check how many are full, and calculate your space full with the results.