The part of your day worth planning
A day has two halves. There is the part that is different every time, meetings and errands and whatever landed in your inbox overnight, and there is the part that is the same most days. A calendar handles the first half well. Almost nothing handles the second half, which is why people keep rewriting the same six items into a fresh planner every morning.
Rewriting them is the problem. If an item appears on your plan five days running and gets done twice, that is information, and copying it forward by hand is exactly what destroys it. Here it stays as a row, and the row keeps its record.