The purpose is that I'm curious about overall larp activity. Player-spaces per month is a crude measure, but an easy one, and at least lets us see large-scale shifts.
People obviously attend multiple games, sometimes multiple games per month. You can measure community size by getting the player lists for every game (or every game of the type you're interested in), stacking them up, sorting by name, and just counting. I do this sort of exercise for the Wellington theatreform market, which has given me useful insights into total player pool and viable games sizes. If you want to do this for the Auckland theatre-style or WoD markets, feel free.