Perhaps, like me, you don't like having iCloud as the default location for all file actions. Disabling iCloud document and data storage will fix this, but you may want to know what you've stored on iCloud first.
You can't do that from the web UI. Instead you have to inspect the hidden local caches:
- How to Access iCloud Documents From the Mac OS X Finder (user library folder / Mobile Documents)
- Access the iOS Photo Stream from the Mac OS X Finder (trickier!, I don't have this folder as I don't use Photo Stream)
Incidentally, you can use iCloud as a general document store to pass files around, but this is unsupported. I used to use DropBox for this, but for various reasons I currently use Google Drive.
 I'm sure someone has experimented with putting a symbolic link to a Google Drive folder inside the iCloud Mobile Documents folder.