Focus is the essential requirement for success. Every success comes from focusing on your goal. The goal may be silly, very small comparative to others but when you focus on it you will be the ruler. So marketing guru Al Ries tells Focusing is key for success.
Not necessarily only in marketing, but for every moment of our life- focus leads to success. I can tell you why. You may be a software engineering student. Instead of trying to become master in every thing that comes in syllabus, just you could focus on "c", a very first language you learnt in your studies could make you master in "c" and you can rule it. If you are a MBA student you can just focus your ideas on McKinsey's 7s model and you can become master and it gives value to your life.
Most of the successful stories follows the same pattern. Google concentrated on search, it is ruling the world of search engines (now they are diverging, that is different kind of story).Coca-cola focused on coke, Mc Donald focused on Hamburger, Pizza Hut concentrated on Pizza, Coffee day on Coffee, Microsoft on Windows..... all are now unquestionable in their field. Today you decide what to focus on. Focus on what you want to focus. You will rule it.