Generally the best times to hit a Purpose Zone are when you are ahead in the count 0-1, 0-2 and 1-2.  Getting ahead will give you the option of throwing a ball in a Purpose Zone to set up an off speed pitch away.  You have more options of pitches to choose from while ahead in the count and the hitter has to protect against all pitches and the whole strike zone.        

There are situations to use this pitch earlier in the count. When a particular hitter has hit you hard earlier in the game the opposite way, pulls an outside pitch, or it is known they like to dive over the plate, it would be a good idea to run a pitch inside 0-0 or 1-0, 1-1, or 2-2, to send a message

Be careful throwing a really aggressive purpose pitch to weak hitters down in the batting order because of the possibility of hitting the batter. Base-runners are base-runners, and it can be especially costly if the runner has speed. It doesn't matter how they reached base when they cross the plate, it still counts as a run every time. 

A good alternative to the aggressive Purpose Pitch is a pitch around the letters of the hitter and over the middle of the plate.  You may be more likely to get a swing and a miss on this pitch compared to the Purpose Pitch.  Throwing the high hard fastball will speed the hitter's bat up enough to make them vulnerable to the off speed pitch down afterwards. 

Don't fall into a pattern of throwing a pitch away from the hitter every time immediately following a Purpose Pitch. Try following a Purpose Pitch with another inside fastball for a strike on the next pitch.  This is called doubling up inside on the hitter and it is a great tool to have in the bag.