To appreciate the advantages of file sharing, consider the following scenario:
Your team develops a suite of tools that do different things, but work on the same data format. They are built around a common set of core code, modules and header files that are used by many programs. You store each tool's files in its own project. You also share the common files among projects so that any change made to a common file is immediately propagated to all the tools.
For example, the database group of the office tools team wants to add a spelling checker. The word processing group already has a spelling checker. The database group could just add the spelling checker files to its project, but the two groups would not be connected. If the database team found a bug in the spelling checker, the word processing group would have to duplicate the change. By sharing the files, both groups work with common source files and all changes are automatically seen by both projects.