For some reason iCloud is taking up over 30 gigabytes of storage on my MacBook Air. How do I get rid of this? I though the whole point of iCloud was to save space not eat it up! I'm on macOS Sierra 10.2
Your iCloud Drive contents are cached. After looking over the new storage management features in Sierra, doesn't look like the "Optimize Storage" feature will clear that.
If you go to About this Mac > Storage > Manage > iCloud Drive are there options there to remove files locally? I don't use it personally so mine's empty. :)
You can remove iCloud storage from your Mac by going to System Preferences → iCloud, then uncheck the checkbox next to iCloud Drive.
Beware that everything will be removed from your Mac but remain in iCloud only. This means that anything added to your Mac after this will not be stored in iCloud and you'll have to do it manually on the web.