The nature of SMS delivery between network operators and mobile devices is such that a store-and-forward methodology is used. Part of this methodology involves holding on to SMS that can't be delivered (most commonly due to the receiving handset being inactive) and re-attempting delivery repeatedly for the duration of the message's life (also known as its 'validity period').
Although setting the expiration date/time to the maximum gives receiving handsets the maximum opportunity to receive messages, the ability to set a validity period that is less than the maximum can be very useful in certain occasions, e.g.: