As someone has said it is a way of spreading a reduction step over more time so the body notices it less. Dropping too much for you personally at one time can cause you to overshoot the dose you need or cause steroid withdrawal pains - which are so similar to the reason you are taking the pred in the first place that you think it is a flare. The smaller the change at any one time, the less the body complains and the quicker it gets used to the new dose.