Aside from gaining a copy of a file one didn’t buy, there are many ways to use file sharing services, that are more legitimate:
Getting tracks from a misplaced CD by downloading them from the Internet. Alternatively some people are too lazy to find the CD in their collection, and instead download it from the Internet.
Getting a digital recording of a file from a purchased audio recording like an audio cassette or a Vinyl record.
Getting the video clip of a song one has bought. These are usually hard to acquire separately.
Try before you buy.
Sharing songs that were declared freely-distributable by their originators.
Getting singles with different variations for albums that were purchased, especially a long time after the release of the singles, when they are very hard to find.
Prohibiting file swapping entirely will, thus be like throwing the baby along with the water. Nevertheless, all of this is secondary to the fact that preventing the non-restricted non-commercial distribution of artworks is bad.