Coincidentally madeinuk's post reminded me... another use of the left column may be to point to other resources. Textbook page numbers might be one example of a reference. At some point, other outside references may also be helpful.

Some may think of the Cornell format as documenting the natural conversation we have with ourselves while we are learning:
they said this...
main ideas were...
this information fits together like this...
this information connects with, parallels, or cross-references to other things in this way...
this raises questions in my mind...
what answers might I find...
etc.