You don't need to limit use of secondary doms to changing keys. You can also do it within a key. Adds depth and a richness of sound to a chord progression that makes it sound more integrated.
For example, the old standby of playing in G major and going to E minor. This is just I to vi - or change to relative minor:
|-3-------0-|
|-3-------0-|
|-0-------0-|
|-0-------2-|
|-2-------2-|
|-3-------0-|
But how about we prepare for the arrival of the E minor chord?
|-3--2--2---0-|
|-3--2--0---0-|
|-0--2--2---0-|
|-0--4--1---2-|
|-2--4--2---2-|
|-3--2------0-|
Notice that even though F#minor and B7 are not in the key of G they can be used very nicely to sound as if you are "temporarily in the key of E minor". Just long enough to get to E minor, then you can just keep playing in G major.
Does that little example sound familar?! Sure, it the opening to "Yesterday" by the Beates - and a million other jazz tunes & classical pieces.
