This is a really simple, bare bones, FREE multitracking program with solid editing features... I have it, sometimes its nice to do simple work in a simple environment. Its also not much of a CPU or RAM hog for slow systems.
I used to use this just for recording my vocals, as well as converting my .wav files to MP3. Recently, I learned about importing raw data, and I've been able to make some great glitch sounds, and even some sounds for noise music. Try it sometime if you're into those kind of sounds.
once upon a time the audacity team saw the midi file format and decided instead of making a sequencer, they should just make their software show you what a midi file looks like instead. basically all you can do is import files, do some sick tricks, and spit it back out as a wave file. what's that? you want mp3 export? you have to download a library manually. it's like installing a vst plugin but a few thousand times more painful.
I know this is primarily a music website, but I feel the need to point out this software. I used to use it for recording my vocals, and then I would export them into Fruity Loops. However, now my vocals are recorded in Fruity Loops itself. However, I still find use in Audacity, just not for my music. I often use it for recording my lines when I do my voice acting, and adding various effects before I export it into Cool Edit Pro for editing. So, yes, this is very good for audio editing and recording, but not really that much else.