Use the time of a total stranger in such a way that he or she will not feel the time was wasted.
Sound advice, but certainly not essential. I think there are ways this can be abused. For instance, I've noticed that sometimes, Stephen King for example, the narration will suddenly switch perspectives, because he wants to bring out the drama, and use a stranger as an observer of something happening to the main character, usually when he's dying or exploding or something. Problem is, the reader is suddenly confused by this switch of perspective. Who's this guy? Why's he important? Should I be paying attention to him? Anyone who gets a scene his/her own perspective registers as 'main' in my head, don't know about you.
At least I think that's what this advice is saying. It might also be saying that you need to write something worthwhile, something satisfying, so that a total stranger enjoys the time reading it. Also good, but I think if you're writing, you automatically want to do a good job. If you don't, you shouldn't be in the business.