Pitches per start tells us how many pitches a starting pitcher throws, on average, in his starts. It can be a useful tool for evaluating pitchers in many ways -- particularly while he's pitching. P/GS gives the viewer an indication of just how much a starting pitcher might have left in the tank.
MLB's leaders in pitches per start are typically veteran pitchers who aren't very injury-prone. Overall skill helps, too, because a manager is obviously more inclined to leave a pitcher in the game longer if he trusts that pitcher to get outs. Young pitchers are generally watched by their clubs, so as to keep their pitch counts down and limit the stress on their arms. The same goes for pitchers who may be prone to arm injuries.
It's important for each team to have at least a couple pitchers with high pitches per start averages, as those pitchers help limit the tax on their teams' bullpens. However, because pitches per start doesn't tell us much about a pitcher's success, it isn't the best way to evaluate how likely a pitcher is to go deep into a ballgame.
In A Call
"pitches per game started," "average pitches as a starter"