During last night's Diamondbacks game, Randy Johnson was saying something to the effect that pitchers these days can't rely on just their fast ball. Today's hitters are too fast and have more analytics and they can take about any fast ball and go yard. The average pitcher today now has to vary the locations of their fastball more today, AND have a greater arsenal of secondary pitches or they're not going to be able to compete.
Drop the mound another inch.
So basically “throw strikes they can’t hit”. I heard that from my coach in high school, and it sounds like the concept still holds true.