If you don’t, you will become exhausted.
Spend your energy wisely. You’re only human, and you don’t have infinite energy. If you try to do too many things, you will become exhausted and burn out. As you become more tired, your productivity drops dramatically. Your mental acuity also suffers, making it harder to solve problems. You will make more mistakes.
Thus, it is crucial to get the right things done. This means you have to say “no” to some tasks, intentionally not doing them, so that you may complete the more important tasks.
This is why it is so important to set priorities and goals. When you enter your day with a plan, you have a lot more confidence that you will be spending time on the most important tasks. Start each day, knowing what the most important goals for the day are. Mark them off your list as you accomplish each one.