Knowing openings means you don't have to spend lots of time trying to understand your opponents moves, since you've already done your homework ahead of time. It also means you understand the general plans for both white and black, the themes, and any traps. Not knowing openings will generally mean that you get an inferior position to work with in the middle game, or you have to spend a lot of your clock time figuring out in a couple hours what people have spent days/weeks/months studying and writing books on.
In the computer era, setting up a good repertoire seems like an easy task, but the truth is that it takes many hours to prepare and then study openings. The Chess Databook 2015 is designed to help you study all key positions that matter. We have worked on our database for over 12 years to make it most complete and easy to learn for you!