Why do people use WASD instead of the arrow keys?

WASD (,AOE on Dvorak keyboards; ZQSD on AZERTY keyboards) is a set of four keys on a QWERTY or QWERTZ computer keyboards which mimics the inverted-T configuration of the arrow keys. … Primarily, WASD is used to account for the fact that the arrow keys are not ergonomic to use in conjunction with a right-handed mouse.