They all have a differant reason for being. Some are for keeping the wrist from bending backwards. Some are for locking the wrist into a certain position. Others help you hang on to the ball longer. I have provided you with a link to some wrist supports. Just click on a few to give you an idea of what they do. http://www.bowling.com/shopping/all/accessories/wrist-supports
If you want one of any of these please support your local pro shop and have them order what you want. I am in no way affiliated with bowling.com or a pro shop.
Best thing is to see a coach. If a coach is not available just ask around the center you bowl at. Now if you know your wrist is going backwards during your release then you might need one. There is some debate on if anyone really needs them or not. I think that they can aid in training fairly new bowlers. Also for either damaged or weak wrist. A lot of bowlers at some time in their life gets a hurt wrist and some of them can take the strain off the wrist.