A file system that will hide certain files whose names match a regular expression. It could choose to hide these files only from certain users. There are times when you wish to provide access to certain directories but only to a few users. Other times you want to totally hide the existence of some directories or files (exams, proprietary mail, salaries, etc.) from anyone but yourself and the operators performing backups.9
More generally, there are times when you want to perform certain operations only on some files, perhaps as few as a single file. Having the ability in a file system to be as granular as one file can be very useful. The main fashion by which this file system operates is when looking up files names, it decides what regular expression matched the file name, and then can classify the file in question as one of several types, each of which can be passed on to be operated upon by a different file system: compressed files can be passed to Gzipfs, encrypted files can get decrypted automatically, and so on.