Many things play into this. Statistics can be illuminating, especially when adjusted to the ballpark a game is being played in. But there are a LOT of stats.
Let's say you are wondering to what to do with a particular hitter. Here are some factors to ponder:
What pitches and locations does this guy hit? Which pitches and locations make him vulnerable?
Is he predictable with his approach or does he change up constantly?
Does he have command of the strike zone or does he chase pitches?
Does he put balls in play or is he strikeout-prone?
Where does he tend to hit the ball?
How well does he hit each of your pitching options?
Does he see the ball well in this park?
How has he been hitting in this particular game scenario (outs in the inning, runners on base, runners in scoring position, so on and so forth)?
How has he been hitting lately?
Can he bunt? Does he run well?
We could go on and on, but the team has an extensive profile of every opposing hitter and an extensive profile of each of their pitchers. It's up to the field staff to assess the data overlay on the fly.