Guides like this cover the basics of your units, when to build your units, how to generate quick wins, and how to take down your opponents without having to play 10 year olds. The idea is that by understanding how your race and those of the other players work, you can start anticipating the tactics your opponents will use way in advance.
Obviously, if you're an old pro who played the beta for four months, this may not be necessary for you, but if you're just getting into Starcraft 2, a guide like Starcraft 2 Mastery is probably the only way you'll be able to truly enjoy the fresh new content that Blizzard has laid before us.