I once read a quote from a famous author on the value of purpose in writing. He said something to the effect that any strong purpose, whether it be a good one or not, is necessary for what we call "greatness" in a manuscript. That, I feel, is not the proper approach -- as I place high value on works of a good purpose and scorn books of bad purpose -- however, I do agree with him that a strong purpose makes a difference in the story.
Sometimes a book's purpose is obvious; other times it is very subtle. Each can be worked for the message that the author wants to get across. Oftentimes the best method is to have some place, tucked inside the story, where the author communicates clearly the purpose hinted at in the plot. It need not be hammered into the minds of the readers; it can be gently included in such a way that the reader must be attentive to ascertain the purpose.
If we carry the analogy of a backbone, you can see that purpose gives the structure to the story. It helps the story to achieve more. Even metaphorically speaking, a person or a story with "no backbone" is considered "wimpy." No one needs to see the backbone in order to know that it is there for it influences everything. And for me, as a reader, a good strong purpose is often the difference between "that was a nice story" and "wow, that was breathtakingly awesome."
Look at the classics -- did they have purposes? Of course, they did. Every literature class, whether you like it or not, makes you search for the great author's "purpose." Why is that?
Purpose is a vital part of your story -- one that is frequently ignored by beginners -- and taking the time to ascertain your own purpose will make a difference in your writing. Try it!