Then the client is served the uncompressed file but only the compressed version is stored on disk. There are many other interesting uses of filters. If a file has a filter only that file will be filtered, not any wrappers or includes . The ability to filter files can be restricted in several ways. The -u option allows only the use of filters owned by the owner of the index.cache file which lists them.
Filters are also in some ways more secure.
The first thing to note is that there is no requirement that the filter program actually make use of the file being filtered. This file must exist though. It is then possible to read the POST ed data by opening and reading the temporary file containing this data. With a filter the name of the executed program is never visible outside the server. It is not in any URL and it is not in any served file.