Why is the iambic pentameter useful in shakespeare's drama macbeth?
a - in a drama the lines are best written in plain verse such as iambic pentameter so everyone can understand them
b - in a drama the lines are best spoken and this meter has all stressed syllables
c - in a play the lines are not spoken so the meter doesn't matter
d - in a drama the lines spoken aloud and this meter follows the nature voice pattern

Shakespearean histories and tragedies are written in iambic pentameter for characters who are Intelligent and regular prose for those who are not. The exception being The Tempest; it's technically a comedy, but it's the only one that's not a history or comedy written in iambic pentameter. When spoken allowed, the words go from being stressed to not stressed. Think of up and down, like the sentence is a wave. The idea is to convey certain meanings or importance. I don't know of any examples for Macbeth, but from Hamlet the famous line goes, 
"To BE or NOT to BE. (pause, for the unstressed beat) THAT is the QUESTion." (all caps represent the stressed words) 

Hamlet is contemplating whether or not he wants to live.