Modes are often refered to for soloing and improvising. It’s more important to realize that each mode contains the same notes as some major scale. A d Dorian mode has the same notes as a C major scale. So, rather than learning a “d dorian mode” on the guitar, you just learn to related it to C major. Example: You’d like to play a d-dorian mode over a D minor chord. Instead of learning an entirely separate scale, you just play a C major scale (which, presumably) you already knew.

Modes also come up when analyzing some music. “Modal mixture” (see here: http://www.classicalguitarblog.net/2009/03/modal-mixture/ ) is borrowing notes from parallel scales. That is, if you’re in C major, you can borrow notes from a C minor (C Aeolian) scale. That would give you a Bb, Eb and Ab to use rather than a B, E and A.

-CD