The progression of a V chord, also called the dominant, to a I chord, also called the tonic, is the foundation of functional tonality. Every chord in the diatonic collection has a function. We teach students what chords “tend” to do. Sometimes V goes to vi and fools us — we call that a deceptive cadence, as we deceive the listener’s expectation.
Composers and audiences alike fatigue of a single key and variety is welcomed. As harmony evolved, the movement to other keys were achieved by somehow getting to that key’s dominant, and then switching over. But sometimes, our old theory teachers tell us, composers JUST GO THERE. In other words, by not using any logical or functional pivot — the composer “goes there” for the new key’s shock value. That shock value changes over time, of course, but the vestiges of those ancient shocks should still be in performances.
I am using a different kind of modulation in the opera I’m writing now (Angela Peralta). As there are a lot of flashbacks, I need moments that feel like musical versions of crossfades, and I often will do this as one key fades, another completely dissonant one fades in, giving the impression of a veil being lifted. And, when we leave that flashback, that tonality evaporates and we find ourselves back in the key we left a few moments earlier.