King Lear - Edgar plays many roles throughout the play and performs a wide range of functions. What is the purpose of his role playing?

After reading King Lear and studying the role of Edgar at some length, it is hard not to be disappointed with the outcome of Edgar's role. He performs so many roles and functions that he seems to just be a 'plot device'. Not much time was spent by Shakespeare on establishing Edgar's qualities and virtues prior to starting him off on his journey of role playing.
Through this role playing it is possible to detect progression in Edgar's character as he moves from one role to another. He grows in stature through his use of disguises. He is forced to assure the garb of a madman to preserve…