The grid screens help you make sure that your image is, or at least appears to be level or vertical or square (as circumstances require).
Originally Posted by Andrew Laverghetta
Some of the other screens are more usable when the circumstances or equipment make it impractical to use split image or microprism focus aids (such as stopped down photography, with shift lenses or bellows). In addition, screen patterns and focus aids are acquired tastes - some people just prefer some choices over others. Whether or not one screen type will appeal to you more than others is hard to predict - it's best, if possible, to try a few, and then decide for yourself.
One other point, if you can change the screen, it means you can easily clean the screen. There is almost nothing more irritating than dust and dirt trapped on a fixed screen (how does it get in there anyways )
The price you paid for the 645E is good. Have fun.