I think sometimes we underestimate how impactful 10 minutes can be. Imagine sitting in a call, waiting for folks who are late to join the meeting, or that's five 2 minute video ads on YouTube - Google is taking 1% of your day in ads.
I think it increases the perceived value of 10 minutes, especially when looking at the visual representation.
Once you account for your own non-negotiable slots it may well be 5 or even 10 percent of your (available) day.
I don't get the same effect when looking at an agenda planner as the typical day to day items are not present. There seems to be time to fill on calendars, the opposite here.