Just knowing what to do when isn't enough for some of us. We want to know why the rules of thumb exist, and what we can do to figure out the solution to new problems. In the case of high and low impedance devices, the details of why can get pretty technical. By understanding them, you can make better informed decisions about gear. For example, do you need an active distribution amplifier with your current setup, or is passively splitting the signal okay?
You can read the full article here: http://www.prosoundweb.com/article/understanding_high_and_low_impedance_signals/P3/