First we have to remember that sfumato is a technique that you get by adding layers of painting extremely thin, giving imprecise edges which, then, helps us to show depth and old image depending on the colors or work you apply to it.
Da Vinci was obsessed with reaching perfection and this applies also to his paintings. This technique also creates a more realistic image due to the dimensions. As a painter you can "play" with how far or close are the objects by making the edges more or less blurred.
If you use this technique when painting the face of a person, you can also create a feeling of "sweetness" or "kindness" as in the Mona lisa where he used it at the edges of the mouth and eyes to get this feeling.
This technique is very important if the intention is creating a realistic and more precise painting
