It’s important for a blogger, or any site owner to scope out other sites in your niche or subject area. While you are poking around, it helps to know if the site you are looking at is well ranked and trafficked. I’ve recently added the Compete toolbar to my browser as well.
So for any given site I’m looking at, I can see the Alexa Rank, Google Page Rank, and Compete Rank at at glance. Note that these ranks aren’t the end all, be all of a given site. But they will give you a good idea where the site or blog fits in the big picture, and a guess at the money they might be making. By many accounts, there are close to 200 MILLION registered domain names, and about 80 MILLION active ones. See where sites you visit fit.