Yeah, that's for their ad-free Premium thing, which gets rid of ads and lets you skip the nag screen you're talking about. Which, fun fact, you can use uBlock Origin and a Greasemonkey script to skip (I'd recommend ViolentMonkey to run the script) ;)

Which reminds me, the top mods every game/month are eligible for a small cut of that money if you opt-in and give them your real personal info. I've only ever made the cut once so I completely forgot about it.