Answer to Question 1
Soil has small pore spaces, but an organic medium has large pore spaces to allow air to reach the roots of the plant. A potting medium will not compact under containerization as soil will. Also a medium will be sterile and not harbor disease organisms, weed seeds, and insects. Finally, potting media are lighter in weight than soil.
Answer to Question 2
organic component to retain water and act as a reservoir for nutrients
coarse aggregate - to create large pore spaces that allow the free flow of water and air through the medium