The basic idea is that some binomial distribution problems -- for example, finding the probability that if you flip a coin 8 times you'll get 5 or more heads -- get really time-consuming for larger numbers of flips (trials). It's the "or more" that gets you. Using z-values makes this type of problem a lot faster and easier!
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)