Synthetic brushes contain less pores than natural brushes so they are flat. Less pores means less permeable to water, air, etc. Since the bristles are less absorbent, they are more ideal for liquid and cream foundations, concealers, cream blushes, cream eye shadows, and lip color. You don’t want a brush that will soak up the liquid or cream and not deposit any on your face.
Natural brushes aka “sable,” are more porous than synthenthic brushes. The bristles are more flexible and ideal for picking up powder blushes, face powder, and powder eyeshadows.