I've got a "developer-old" folder besides for my "developer" folder. "developer-old" contains "headers", "SDKs", etc folders.
It'd be nice to delete this folder bc it takes up 10GB. Can I?
Yes, you don't have to keep that folder according to https://discussions.apple.com/thread/2780986?start=0&tstart=0
It's the backup of old Xcode installation.