Assuming that you have the cleats positioned correctly, float supposedly prevents injuries.

This is from Speedplay's website:

Why is "float" important?
The float in X pedals helps to prevent the type of chronic knee joint stress that occurs when a cyclist's foot is held in a wrong position. X pedals adapt to the cyclist's biomechanics, preventing joint misalignment and overuse injuries.

Why do X pedals have such a wide range of float?
The wide range of float is a necessary design by-product of a free floating pedal system. For safety reasons, a free-floating pedal design requires a release angle greater than fixed pedal designs. Although the majority of riders will use only a few degrees of the available float, riders with serious biomechanical misalignments will use more. X pedals do not ever cause a rider's foot to use the full range of float. A rider naturally uses only the float needed to prevent misalignment.

Is there any loss of power from pedals with rotational float?
No. Several independent studies have concluded there is no loss of power from float, yet float can be instrumental in preventing repetitive-motion injuries due to misaligned joints.