In some case the photographer will need to reschedule with you for a slightly different time on the same day due to prior commitments. It is rare that a photographer would need to reschedule for a different day but it is a possibility.
In addition, photographers are keenly aware of lighting and getting the best shoot possible. Sometimes photographers will want to adjust the schedule to get the very best lighting. Also, as for on-location shoots weather can be a factor and need to be rescheduled if the photographer believe it will result in a sub-optimal photo shoot.