Because eventually you have more things to do than be easily remembered. Carrying them around in your head is both fallible and stressful. Diarizing a to-do for

Because eventually you have more things to do than be easily remembered. Carrying them around in your head is both fallible and stressful. Diarizing a to-do for three months in the future and not worrying about until the day it comes up is a much more robust system than relying on your memory.