Why do we paint the Still Life? What can be learned?

What can be learned from painting a still life besides the fact that it makes a good subject and has inspired artists throughout the centuries?
First and foremost we are studying FORM. The shading on the Egg, the Sphere, the Cube and the Cylinder are all forms of which the professional artist has an intimate knowledge. 

We study the way light falls across these forms in revealing their essential elements.So the two most important questions we must ask ourselves are: WHERE IS THE LIGHT COMING FROM? and WHAT IS THE FORM WE'RE DRAWING or PAINTING?     ( Of course the light source must come from one direction.) These 'essential elements' are THE FIVE PLANES OF LIGHT; Light, Shadow, Half-tone, Reflected Light and Highlight.  
We're learning to "see" as we study the concepts and principles of art. From painting the still life we're also studying the qualities of various textures including glass, silver, copper, porcelein ,various materials (cotton, velvet, satin),fruit, flowers and more. At the same time we're learning techniques and how to apply paint; transparencies vs opacities and COLOR. We study the nuances of mixing colors; cool blues and warm blues ;cool reds and warm reds; cool yellows and warm yellows and so on.
WE study VALUES ( lights, middle tones and darks).
To simplify , this is what we study and learn from the still life:

• Composition 
• Form
• Light and Shade
• Textures
• Color
• Values
• Space, Light and Atmosphere
• Techniques

An Egg shape is found in a lemon, a tree and the human head; A sphere is an apple or a peony or a brass bowl; A cylinder is a bottle, a cup or the human neck; A cube is a book, a building or the blocking of the ribcage.
So painting and understanding the still life makes the portrait better as well as the landscape.