One of the keys to good pitch selection the ability to read a hitters timing. One way to get information on a hitters timing is the foul balls hit during an at bat.
If a right handed hitter is fouling fastballs down the left field line then his timing is early. If he is fouling fastballs down the right field line then his timing is late. If the foul ball goes straight back his timing is good.
Hitters who are early and pulling fastballs foul are good candidates for a change up on the following pitch. The reason a change up is generally a better option than a breaking ball is because it has the same spin and ball flight as the fastball.
Hitters who are late on fastballs are not good usually good candidates for anything off speed on the following pitch. If a slower pitch is thrown then it is giving them more time to get the bat head out and time the pitch.
Timing is good on swings when hitters foul a pitch straight back. Generally they are not happy they didn't hit the ball hard and fair in this situation. Pitch selection on the following pitch usually calls for a change of speeds or at least better location.
Pay attention to where foul balls go and what you can do to adjust accordingly. These in game adjustments could be the difference between giving up a three run homer and getting a double play to end the inning.