Before the age of realism characters could reveal their thoughts directly to the audience by means of a soliloquy. This sidekick or buddy is there to reveal his innermost thoughts to for the benefit of the audience. These characters also provide a handy norm against which the relative goodness or evil, wackiness or normalcy of the central character to be gauged. Finally there are the minor characters who round out the telling of the tale. These folks have at the most one good brief scene, or at the least a line or two.
All dramatic characters are exaggerated versions of real people.
The degree of exaggeration depends on the type of drama. In serious realistic drama this exaggeration is very subtle. We say the same thing several different ways in our struggle to be understood. But even when dramatic characters appear to be inarticulate, it is only an illusion. They are actually communicating a lot of information quickly in order to keep the plot in motion.