Well
If the machine went into spin full of water it would cause unnecessary strain on the motor, also the machine would be inclined to become out of balance if the clothes were not evenly distributed around the drum prior to spinning.
To prevent this the machine will rotate at a speed high enough to distribute the clothes around the sides of the drum until all the water has drained out so that the machine will be more stable and less strain will be put on the motor during the start of the spin.
Matt