I figured it was for switching keys, but I was too afraid to press it in case it freaked out and I could never play again. Yeah, I only own it on steam and I use a .bat to launch it.
start "" "Blockland.exe" ptlaaxobimwroe
It still gives me the steam login thing though. I'm pretty sure.
I've only tried it with a LAN server, the reason I use a .bat is because I can't launch through steam with a dedicated server running. I host a dedicated server and play on it on the same machine with Steam, the dedi server just has -dedicated in the .bat as well.