How do deviations help the literary artist to convey his/her message to their readers?
I find this question quite puzzling. A deviation is typically defined by an action which departs from a norm, purpose, or subject. That being said, a message will not be able to be conveyed if an author turns their attention away from it.
When an author determines a specific message to be examined, explained, or identified moving away from said message will fail to allow the reader to understand the subject fully. An author who uses the "meat" of his/her text to define a message is the one who does not alienate their reader.
Authors who stray from their initial message tend to alienate the reader by sending them down one path only for it to stop at a dead end. The reader then leaves the text feeling betrayed and confused.
I cannot think of any authors who have deviated from the meaning so as to provide a textual example for your question. Authors who would tend to do this would certainly not produce many works, let alone works which be become renowned.