In the case of Dodge, DB exists for the same reason why diving for cover gives a bonus: you're moving behind an obstacle as you dodge, allowing a less-complete void to serve the purpose of more radical movement.

COVER
To take cover behind an obstacle, simply move so that it is between you and your attacker. You might also have to kneel or lie prone, depending on the obstacle’s height. Cover protects one or more hit locations, making you harder to hit with ranged weapons.