When to Completely Drain the System?

Sometimes when I have called a plumber, he has asked to turn off the mains water at the stopcock AND completely drain the system by opening all the taps. At other times, it appears it's necessary ONLY to turn off at the mains and isolate a valve. I want to understand my plumbing system and would appreciate knowing WHY it is sometimes necessary to open all the taps and drain off all the water in the system (which is of course quite wasteful), and sometimes not. There are usually isolater vallves I believe, so why the need to drain the system? I would really appreciate being able to understand. Thank you.