The purpose is to specify the rules for an interoperable PHP autoloader that maps namespaces to file system paths, and that can co-exist with any other SPL registered autoloader. This would be an addition to, not a replacement for, PSR-0.
The current autoloading standard definition (PSR-0) still allows for the use of the underscore in class names to resolve to directory paths in the application's files. In this new standard, that allowance is gone, relying only on the actual namespacing to define package pathing. This "package-oriented autoloading" is set to help move PHP package development forward into a more standardized structure.