The reason some vehicles have steerable rear wheels is because there are two rotational effects required when turning. One is to make the vehicle rotate, the o

The reason some vehicles have steerable rear wheels is because there are two rotational effects required when turning. One is to make the vehicle rotate, the other is to make the vehicle move in a circle. If you only steer the front wheels then when the centripetal force is correct for turning the corner, the rotational force is much larger than necessary. This leads to excessive scrubbing of the tires. Making the rear wheels steer slightly in the direction of turn the rotational force is reduced, similarly reducing the scrubbing. As a side benefit the wheels can be turned in the opposite direction at low speed, increasing the manoeuvrability.