Christopher, Master tracks control the output level being fed to an output bus. Most often, a master track will control the main left and right outputs for a stereo mix. It's not necessary to necessarily use a master track for a regular mix but it certainly does help to create a master track to see the combined level of all of the individual tracks on the master track's level meter. Further, when putting bus compression over an entire mix, it would normally be done with a compressor plug-in on a master track. Finally, master tracks are used when creating stems for surround mixes, alternate mixes, instrumental versions, etc. Again, most people just use a single master track for overall mix purposes.
HTH, Slau
