Having to start from scratch creating multiple similar but not identical filters is a real PITA. Another scenario is when a new need arises and the new filter only varies slightly (but enough to require a new filter) from an already existing filter you've written. If you could copy a previously created filter that was close to what you wanted for the new filter and then modify it as necessary for your new use, it would save a whole lot of time and reduce the number of errors in those newly created filters.