Why Insenia asks this
Every evening starts with the same slider, worded the same way, so that each answer can be placed next to the ones before it. A single day on its own says little, and it is only in the row of days that a number begins to mean something.
The question asks how the day felt on the inside, not what happened in it. The answer is yours and nobody corrects it, a 3 is a 3 because that is how it felt.
What it tends to reveal
After two weeks the row of evenings starts to show a shape. Some people see a weekday that is consistently heavier than the others, or weekends that sit a little higher than the days around them.
Over a longer stretch the day colours in the calendar follow this one answer, so a run of lighter or heavier days becomes visible at a glance. The app describes what the row looks like and leaves the reading of it to you.