Shakespeare mixes verse and prose in his writings. He uses verse mostly to express emotions, to make wise comments on the nature of man, to interject irony and when juxtaposed with prose, to focus on the action that is taking place.
For example, in Act 1, Scene 1, many characters of the house of Capulet and Montague appear in the street and have various arguments and fights, but then the prince enters, and his first speech stands out in very precise iambic pentameter. It gets your attention when you hear these words spoken because they are regular and rhythmic as opposed to the dialogue that precedes it. Remember, this is a play, intended to be viewed and listened to, not read. So the sound of the regular iambic pentameter is very important in Shakespear's plays. Language, its sounds and rhythms, were his only "sound track" so to speak.
As you read the rest of Act 1, look for examples of iambic pentameter and then ask what Shakespeare is trying to accomplish in that particular section - emotion, action, wisdom - and you will be able to answer this question for yourself. Hope this helps!