Importance of routine for a dog
If you do things the same way over and over, a dog learns what to expect and is reassured by this. Your dog will be happier and more relaxed if he knows that he will be fed at about the same time every day than if he expects dinner at a time he is used to, doesn't get it when he expects it, worries that maybe there won't be any dinner, and finally is fed, but only after he has felt hunger pangs and worried for a few hours.
Changing the schedule once in a while won't do irreparable harm. Suppose you are late coming home one day. It won't kill your dog to be fed a bit later, especially if you reassure him with words and pats that you didn't mean to upset him.
But in general, try to adhere to a schedule that is fairly consistent. This is especially important if you are rehabilitating a frightened or insecure dog (such as some dogs adopted from animal shelters, or dogs who have been abused), as routine makes all dogs feel more secure. Routine shows insecure dogs that there is a pattern to their life with you rather than a jumble of events and frightening happenings.