Message scheduling is an act of specifying the date and time of delivery of a given SMS. This feature gives you the liberty to differ sending your SMS for immediately if you do not wish to do so.
Sometimes, certain message are better received at specific date and time. The ability to get those messages across at that particular may serve the purpose or give the SMS much more relevance.
Consider a birthday message to a friend. Being able to deliver the message at exactly 12am on the birthday makes the message more relevant as it may likely come as the first message for the day.