Shakespeare wrote in iambic pentameter for a number of reasons, not the least of which was that it was popular at the time.

The iambic pentameter da-DA-da-Da-da-DA-da-DA-da-DA makes it nearly impossible to put emphasis on the wrong syllable. e.g. “Give me a coke a burger and some fries”

Perect iambic pentameter, and no syllables sound wrong.

Tragic characters speak in blank verse not only to make them hearable in a large noisy house, but to accent the important words, and to aid in memorization.

When Shakespeare deviated from the iambic pentameter it was always intentional and to make a point.

The speech of ‘Is this a dagger that I see before me’
‘The handle toward my hand? Come let me clutch thee’

Contains 11, not 10 syllables, and is to show Macbeth’s indecision, since the extra syllables throw off the rhythm.
Hamlet does the same in “To Be or not To Be”

The wierd sister’s chants are not in IP either, but in sets of 8, to show their other-worldliness.

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