That and combat style both.

With dual wield it is all about dancing around the edges of your opponent's reach, jumpin in and landing some deadly attacks very quickly and either dropping your foe right there or kiting by to avoid being hit and then coming around again for another pass with your whirling blades. You cannot block with dual wield so you do not want to stay within their stiking distance for very long.

With Sword and Board it is all about blocking and staggering with shield bash and then timing some good swings before you put your shield back up and block/bash some more. You spend a lot more time up close and personal with your foes. In fact you want to be as close as possible a lot of times so you can connect with your shield bashes to get the stagger.

Since the combat styles are so different, it might be jarring to switch back and forth between them in one character. At least for me it would be. Once I get used to a certain technique, it takes me a little while to get back into some other technique.

EDIT: It is also just fun to have several different characters with different playstyles/perks.