Why does the original pronunciation (OP) of Shakespeare's words matter? For one thing, two-thirds of Shakespeare's sonnets have rhymes that don't work in modern English. 

The modern pronunciation of certain words also robs them of meaning. For instance, it matters how the word "loins" (from Romeo and Juliet) is pronounced in OP because the word is used as a pun on the word "lines," which was pronounced the same way. So the line ("From forth the fatal loins of these two foes") has two meanings: genealogical lines on the one hand and physical loins on the other. 

Shakespeare, of course, was writing at a time when the English language was rapidly evolving. But it wasn't just the words that were evolving, it was the way the words sounded as well. 

This is wonderfully explained in the video below by Open University.

Watch here:

(H/t Josh Jones)