απο xda:
When on CM 11, /data/media/ is the directory for the original sdcard and the 0 is the emulated storage for CM 11. If you backup to the internal, your backups can be in /data/media/0/ or /data/media/. As said before, all the other folders and files are left over from before putting CM 11 on
Internal storage is reserved for apps and data, that means dumb apps won't use internal storage for stuff that could be on SD Card. Basically CM11 fixes the stupid thing when apps downloaded /obb and stuff like that to internal storage, thus hogging it up.
I'll simplify it a bit. System uses entire internal storage for the apps you have installed (and their data). User(aka you) only gets to store stuff like movies and music on External SD, not like you could previously on Stock roms.
Stock ROM:
1) Storage for apps = internal storage - movies, music, app extension files(/obb) and other things. Usually it runs out quite quickly because games are almost always 2GB+. (That means 2 games max and no space for anything else)
2) External SD card was not usable by apps, and it was mainly unused and the space was wasted, because apps couldn't use it.
CM11
1) Storage for apps = entire internal storage. 4GB for apps! A lot of apps can be downloaded.
2) Stuff that belongs to External SD is now where it should be. Movies, music and /obb is now on the external storage where it should be.
If you install a 2GB+ game on CM11 it only uses 30-40MB of internal storage and stores most of the files (~2GB of game resources) on external SD. If you have a 32GB SD Card you can install ~10 games that are like that.