Answer: To show the difference between an educated, refined noble class and coarse, crude commoners. However, this doesn't mean that Shakespeare resented the commoners. It was a self-explanatory fact that the nobles and educated were refined enough to use iambic pentameter in talking to each other - but this has many exceptions too; there are many moments in his plays when the nobles use prose or blank verse, for example when they are chatting to each other or are intoxicated. However, Shakespeare's blank verse and prose, though devoid of contemporary poetic mannerisms, are extremely witty and rich in meaning and associations.