In the course of learning about and experimenting with audio cleanup with Cool Edit 2000, I found myself creating a lot of filters that pass only a single note and its harmonics. It took lots of time to create a filter with many harmonics, and I didn't always get what I wanted because the GUI for editing filters is imprecise.
Creating the filters by hand got me the precision I wanted but the process was still slow. To make filters quickly, I had to write some software.