"No discussion between two persons can be of any use, until each knows clearly what it is that the other asserts" (Lewis Carroll).
WHEN WE DISCUSS THINGS, WE OFTEN ERR BY TALKING WHEN WE SHOULD BE LISTENING. Taking it for granted that we understand what the other person is saying, we're primarily concerned with whether they understand what we are saying to them. Understanding is not as important to us as being understood, and so our discussions often fizzle out ineffectively. What could have been a dialogue between two inquirers, and therefore an exercise in understanding, becomes a pair of monologues between two talkers, both of whom are in a defensive crouch rather than a learning posture.
Our English word "discuss" comes from a compound Latin verb: dis- ("apart") + quatere ("to shake"). It means, literally, to shake apart or break up. But the thing "shaken apart" is not one's counterpart in the discussion -- it's the subject being discussed! To discuss something means to examine it closely by exchanging ideas and viewpoints. When two people discuss a matter, they speak to one another about it in an effort to ascertain truth or reach agreement. A discussion is a "talking over" of something. It's a consideration of a topic by means of conversation.
Discussion is a great help in clarifying our thinking. "Reading makes a full man, meditation a profound man, discourse a clear man" (Benjamin Franklin). We learn not just by thinking but by conversing, and most of us need to go through the give-and-take of a few discussions before we can see a subject clearly. "As iron sharpens iron, so a man sharpens the countenance of his friend" (Book of Proverbs).
But there is another, more important, reason why discussions are valuable: they help us along the path to common understanding. It is by discussing things that groups of people meld their visions into a shared vision, and their commitments into mutual commitments.
Discussions can sometimes turn contentious, as we all know, but they don't have to do so. And when they're conducted respectfully, as among friends, they are truly one of life's great joys. Debates and defenses have their place, no doubt, but discussions have theirs too.
"The more the pleasures of the body fade away, the greater to me is the pleasure and charm of conversation" (Plato).