Pitch usage is a very common statistic to see on baseball broadcasts nowadays as a way of giving fans an understanding of what type of pitcher is on the mound, what his stuff looks like, and what batters will be looking for. The significance of this can be as simple as whether a pitcher is a power pitcher or a finesse pitcher, or it can be as in depth as looking to see how pitchers tend to setup batters in an attempt to get strikeouts or field outs.
The most simple way we could think of for going about looking at this was simply whether a pitch was thrown in the zone or out of the zone! This can give us an idea of 1) if the pitcher trying to throw called strikes or not and 2) is the pitcher trying to get strikeouts. This can tell us what tends to be relied upon more for setting up vs finishing batters.