Without a gyroscope, the Wii Remote could only detect half the possible axis of movement. This is know as 'six degrees of freedom' and is the standard for VR controllers today. Both together can measure any movement within 3D space.
Gyroscopes measure rotational motion, which is the angular velocity of an object as it rotates around a point (the center of the gyro). Accelerometers measure translational motion, specifically the change of velocity of an object as it travels from one point to another in space.
