Story mode and Classic gallery I guess. But since the game is not on steam yet, spending enough on Patreon to get the Steam key will get you the game on steam and should get you access to the harem mode.I know that nobody knows for sure, yet, but assuming Galatea/Harem will be Patreon exclusive, what's the point of buying the game on Steam, then?
AFAIK from what Project Helius has said so far, you need a Patreon account but not an active pledge. That could mean you must've donated X amount in the past, been a patron at one point, or just have an account (I'm guessing been a patron at one point).