In economics, there's a concept called "opportunity cost." It shows the importance of seeing an "alternative gain" when making a decision.
A relative example is that the opportunity cost of paying for a 4-year college degree is earning a salary from working for a firm at that time.
Time management has an opportunity cost. When you choose a time block to do something, the alternative cost is doing something else that is more productive.
Doing both things at once is called multitasking, and the results aren't as good.
A good time manager knows that most activities don't give him much benefit, and only a few can give him the most.
Sometimes it seems like the person is doing a lot, but what is he not doing?