First: plays are written to be performed. This may seem a tautology, but consider: description of the character's eye colour, hair colour, history and rationale cannot be performed. An actor can perform only a physical action. Any stage description more abstract than she takes out a revolver cannot be performed. Try it.
Second: in a good play, the character's intentions are conveyed to the actor, through him to his antagonist, and through them, to the audience, through the words he speaks. Any dialogue that is not calculated to advance the intentions of the character (in the case of Othello, for instance, to find out if his wife is cheating on him) is pointless. If the dialogue does not advance the objective of the character, then why would he say it?
The character in the play wants something from someone else on stage; in this, he is like the appliance salesman. The prospect comes into the appliance store, and the salesman has a severely limited amount of time in which to convince him to make a purchase. Any dialogue on the salesman's part that does not tend clearly toward closing the sale is worse than wasted: it is destructive. The prospect, just like the audience, once allowed to revert to his previous state of inattention, is lost for ever.
What of dramatic poetry? Well and good. It is my contention that drama is essentially a poetic form - that the dramatic line should be written to convince primarily through its rhyme and rhythm and only secondarily, if at all, through an appeal to reason. Note that the truly determined individual - swain, salesman, discovered adulterer etc - confects spontaneous poetry. All sounds he utters are directed towards winning his point; and his speech, should reason desert him, will devolve to a pre-literate poetry of pure intention.