This spell enhances the casters movement capabilities in extraplanar settings by attuning him to his new surroundings. While very few 1st-level priests find themselves in this situation, higher level characters often make use of this spell. Astral celerity doubles the characters movement rate on the Astral Plane; normally, characters move at a rate of 30 times their Intelligence score in feet per round, but this spell increases this to 60 feet times their Intelligence score. As an incidental benefit, the caster also attunes himself to the plane much faster and suffers no penalties for missile fire while astral.
While astral celerity is most often used in the astral plane, it also offers a small benefit to ethereal characters, too: their movement rates are increased by 50%, so a character with a movement rate of 12 would enjoy a movement rate of 18 while this spell was in effect. Of course, time and distance have little meaning in the overall scheme of the Astral or Ethereal Planes, but relative speed could be very important in avoiding an unpleasant encounter or escaping from pursuit of some kind.