Right Brain Draw

From the Van Gogh Museum, Netherlands...Pencil & Black Chalk Michelangelo   Why Do Artist Sketch?  How do they develop their skills/abilites and perfect their desired outcome?

I recently learned of this Artist... love how he didnt NEED to render the Arm for its form but rather suggested the activity of the limb in this self portrait!

Do we simply draw for the outcome? Can the exercise itself become a work of Art? Sometimes....