You ever see that kid with a really high pocket that probably doesn't throw and ask why its strung like that. Chances are he couldn't give you a straight answer. The reason being because he doesn't even know himself. A lot of people don't know the importance behind the pocket placement in their sticks. People think high pockets are awesome and will make you shoot so much faster, but if you have no clue how to adjust for the way it performs, it will detrimental to your game.
A high pocket is mostly what offensive middies and some attacks use. The ball stays nice and high in the stick ready for a quick feed to the crease or a quick step down shot. A high pocket should be used by a player that wants the ball in their stick for the shortest amount of time. Defenders use them as well for long clear balls and quick passes so the ball doesn't take so long to get out of their stick. The downside of a high pocket is that it is very hard to one handed cradle with it. Because the ball is so high its hard for it to drop down and still have room to be secure in the bottom half of the stick.
A mid pocket is a pocket that can be used by anyone really. A middie, attack, defenseman can use them. They are the most versatile and perform the best in my opinion. You don't have to worry about taking one hand off the stick or anything like that. The release is still pretty quick and with most heads you will get the best looking pocket with a mid.
A low pocket should really only be used by a power attackman that cradles one handed often. A low pocket will hug the ball in the lower part of the head and take a long time to release. Lower pockets tend to have tighter channels so this adds a little extra drag to the ball too. These are not my favorite pocket because of how long it takes to get the ball out of the stick, but if you're a player that initiates contact frequently, then this type of pocket is right for you.