Scenario 1: Browsing knitting patterns. What will the finished product be like? Will it drape loosely, or be thick and squooshy? You can read the yarn stats and examine the pictures, but being able to compare it with your benchmark yarn will give you a gut-level understanding of what kind of garment the pattern is going to produce.
Scenario 2: Buying a new yarn. It's easy to get overwhelmed at the yarn store (it's the wool fumes). We've all been there - you pick up a beautiful skein of yarn and stare at it adoringly. You want it, but what would you knit with it? It can be surprisingly hard to evaluate a yarn's real weight when you're in that shopping frenzy. And shopping online, of course, can be even more difficult.