Here's a horrible analogy, since I love them. We might think of an embryonic stem cell as being the letter A, and able to be turned into any A word from Abracadabra to Alakazam. If you can get a stem cell that's already to the point of "Ac" that's not as useful, though you can still find things to do with it if you need Action, Accurate, or Acme. If you need Advil, though, you're screwed.