Because maybe you have a good all around player that is good at any number of positions including catcher, but a girl that is also good at catcher but not so good at other positions but has a big bat. You use the girl good at more positions as the DP because she can freely go in and out of all of the defensive positions except the Flex with nothing happening to the lineup. You dont even have to report it to the umpire except if they are playing catcher or pitcher for courtesy runner reasons.