Why does the essence question matter?

Read several articles lately on the Protagoras and Meno which rightly (IMO) say that Socrates is really interested in the what-is question (ie, the answer that says what the essence of a thing is) and not as interested in the what-is-one-of-those question (unless it helps him to answer the question about the essence of the thing). Why is knowing the essence of a thing so important to Socrates?