What effect does shakespeares uneven use of iambic pentameter have on the reader?

Certain lines stand out, making them seem more important.

By breaking the steady cadence and lulling heartbeat rhythm of lub-DUB, lub-DUB every two syllables, Shakespeare intentionally creates a dissonance that causes hearers' ears to perk up and take notice. It works the same way in song. When a musician suddenly syncopates the rhythm or steps out of sync, it causes people to suddenly listen. And, like good musicians, Shakespeare always found a way to resolve the dissonance, the syncope, after he had made his point, after he had made heard what he wanted to be sure was heard, and pick the rhythm up again.