Why does a change in brightness show that you are looking at two stars not one?

In the case of an occulting binary star system. You have two stars that orbit each other. When one star passes between the other star and the observer it blocks (or occults) the farther one thus decreasing the amount of light to the observer by the brilliance of the farther star. This only happen if both stars and the observer are so aligned to end up on the same straight line at some point.