Secondary dominants are essential to gaining a deep understanding of harmony. 

Check them out in action here on "Green Dolphin Street."

I'm a huge admirer and fan of guitarist Peter Bernstein.

Here's a guarantee…if you practice this line in every key, you'll gain a much deeper understanding of how to play and hear tritone substitutions! This short line is packed with so much language, and he doesn't use one altered note.

Check out this Sonny Stitt line from his solo on "The Night Has A Thousand Eyes!" This is what he masterfully plays over the last eight bars of the song form (II-V-I-VI7-II-V-I).

As always, learn the whole phrase, then break it up into smaller bits.  ALL 12 KEYS :)