In mathematics, the terms “necessary” and “sufficient” have technical meanings. These terms come about when looking at two statements P and Q. If we say that P is sufficient for Q, then that means if P is true, Q automatically has to be true (P implies Q). On the other hand, if P is only necessary for Q, having P be true doesn’t mean Q has to be true (but the other way works, so Q implies P). If we have the P is both necessary and sufficient for Q, that means having one gives us the other for free. They are tied together and are inseparable.
As a student, there’s no shortage of things I could be doing to help my academic career. I could do some side research, I could read more about my field, I could network with other researchers, I could study more for my upcoming exams, I could work through another textbook, I could volunteer for any number of academic events, and the list goes on. There are so many things I could be doing to advance my career and invest in my future that I could be busy every day for the rest of my life. If I wanted to, I could fill my schedule up with these activities and never be done.