It's called "variaton of pace".
Sampras was a master of it ~ I remember a commentator pointing it out during his 1990 Grand Slam Cup demolition of Brad Gilbert; keep hitting it back passively until you get an opportunity, then step up, take the ball early and flat, and catch out your opponent with the change-up.
Pete was more assertive about it than Murray, but then the Scot is much more of a baseliner by nature. I still see him winning plenty of rallies by suddenly injecting pace into a rally, forcing an error from the opponent or hitting an outright winner. It's a far more effective way to success than the Gonzalez/Verdasco school of "thump it as hard as you can every single shot".
He's also one of the better exponents of the lob in the last ten years, though not as good as someone like Hewitt. Plenty of variety for me, especially since the Lendl-ized Murray of 2012 became a bit more aggressive.