It's a "diary" sticky note that I sometimes write on. The set includes three types of sticky notes of different sizes, so you can write in your diary as much as you like depending on your mood that day.
Because it's a sticky note rather than a notebook, you can quickly write on it whenever you think of it, even if you don't do it every day, and you can paste it anywhere you like.
You can also write the date and day of the week, so you can use it as a TODO list.
