Many successful people claim that prioritizing is the most important technique that makes them so efficient at what they do.
It is a given that to be successful and competitive, it is impossible to do and learn everything we want. To optimize our performance we can only choose to do the most of what's most important. In fact, we do this all the time. If there were an assignment worth 10% and a midterm worth 20%, both use 10 hours (which is all the time we have), most of us would choose to study the midterm and forego or put very little effort into the assignment.
That was an easier example, but our lives are full of tough endeavours that require our full attention and sacrifice performance in other areas. Having a set of goals and prioritizing accordingly will make the choices of what to embrace and what to sacrifice easier, allowing you to focus and gain the most reward from an area that MATTERS to you.
Prioritizing is useless if our executions don't reflect those priorities. We sometimes attend to something because it is urgent and forget those that are not. We check our incoming emails, pickup phone calls, even when sometimes they are not the most important thing at the moment. We neglect physical exercises or quality time with loved ones because they may not have immediate effect although they might be very important.
So: set goals, prioritize, focus, and execute.