This is used here mainly as a way of introducing the dorian scale in the short 12 bar format we are using throughout Vol 1, and to make the transition to minor blues which is coming easier. It is also preparing you for major tunes that use modal interchange on the IV chord. For example, the first couple of chords in “Just Friends”, the 3rd and 4th measure of “Misty” and I could go on and on. Although it is much used as a re-harmonization concept to spice up the harmony during the solos in blues, I can’t think of any immediate examples that use it over the head of a blues. I will search my library and see if I find anything.