Once you have a bunch of different instrument tracks in a music project, one really cool thing you can do is create sub-mixes to help simplify and improve the mix. In the old days, you had to have a mixer with more than just the master buss (left and right channel outputs) or at least have the ability to send the outputs of tracks to other tracks. For example, if you had 4 guitars on tracks 1, 2, 3, and 4, you could route them to stereo track 12 (or a pair of mono tracks, say 11 and 12). Then you could control volume and effects for all the guitars with 1 (or 2) slider.
This is soooo much easier with computers and multi-track software. Below is a video from WinkSound demonstrating how to do this in Pro Tools. You can do the same thing very easily in Reaper, where you can use auxiliary sends like in this video, or use track folders.
See the Pro Tools video below: