In a modern desktop environment, users have to deal with files that are available in different ways. For example, files can be remotely available from a Web or FTP site. Sometimes they are stored locally, but are contained in tar or zip files. Sometimes they are stored in a compressed form.
Each of these cases usually requires the user to use a different tool; using different tools means that users have to learn the user interface for each of them, and have to manually make these tools talk to the applications, for example by using temporary files.
To avoid this problem, there should be a global file system namespace that all applications can use to access these different kinds of files. Users should not need to install and use different programs to access files available through different methods, and all the applications should be able to access this global file system namespace.
So basically we need a consistent API that all the applications can use to access these files in a simple way.