It's hard to say if an application like RapidPreview is really that necessary. After all, you can just as much browse for music files and listen to them from your finder.
However, this application has a nice tree view, which only shows sound files, and that you can refresh any time. The find as you type search bar is quite handy to quickly find a song.
RapidPreview also allows you to to save files as Quicktime media links.
The bottom window of the program's interface is a wave viewer. Oddly enough, it doesn't launch when you play a track in the top part of RapidPreview's interface.
You'll have to drag your track to it to see it's wave form.
While not an essential Mac application, RapidPreview makes for a simple audio file browser.