The primary use is in manual AG attacks which is a good backup skill but CCIP/CCRP in practice is of course way better. Maybe practicing weapon delivery profiles it could help to confirm things are where they are expected.
A depressible piper was THE way to get reasonably accurate weapon deliveries before the advent of CCIP/CCRP/etc to help out. The short version is that for each weapon there was as long list of delivery profiles that involved dive angle, speed, release altitude, etc. For each profile there was a depression setting you'd use to get a spot on hit. Now add in things like the day being a non-standard temp/pressure, wind, etc and there's a bunch more factors to add in too. To be a truly sierra-hotel mud mover you had to memorize a whole lot of data, and be able to crunch it on the fly while flying.
For the rest of us, memorize a few standard delivery profiles (30 degree dive, 300 knots, 10k foot release, etc) for the most common weapons and their piper settings, and you'll be reasonably effective. You can use this skill in the advent you have a computer failure/damage. Also you can do it this way if you really want to challenge yourself.
-Jenrick