This feature is useful in a number of circumstances:

You are using Home Studio 9.0 and need to reduce your track count to make room for new tracks.
You have a stereo audio track and you wish to split it to two mono tracks.
You wish to mix your existing audio tracks down to a single stereo or two mono tracks, allowing you to add effects, EQ,
normalize, or otherwise process the audio just prior to exporting as a Wave, MP3, RealAudio, or Windows Media file.
You need to reduce you total audio track count to allow your PC to play all the audio data without problems.

To bounce your tracks, simply choose Tools | Mixdown Audio | Bounce to Track(s). Select the options you like, and click OK.
Remember, all unmuted audio tracks will be included in the Bounce command.