Back in the Stone Age, before there were workshops, it was a very difficult idea to get into musical theatre. Normally, you would be a chorus girl or boy and write something. People would get their start as rehearsal pianists or dance assistants.
What you get at the BMI Workshop is the rarest commodity in New York City: Friendly criticism; people who genuinely root for you; and a chance to rewrite your work, try it again, and hone your craft.