how multi track recording is performed?
Most of recordings nowadays are being done in stereo - two channels. So when you do a recording in studio, then most of tracks are mono (i.e. microphones, guitars). The main job of sound engineer is to place them in panorama (Left - Centre - Right) to create a nice spatial feeling. For this purpose you have pan knob to set this mono track to the left or right speaker (make it more louder or quieter in either of them). Many tracks are useful when you want to apply different filtering to different instruments or add some reverb, change the volume, etc. In the end all waveforms are being summed and voila!

The main track is obviously a stereo track based on your adjustments of track's pan - that's why you have two channels loaded into your MATLAB.