Weekend markets are the time when interesting questions like this come up between you and I; honestly, I am really missing those moments right now. It amazes me how little we as a society know about our skin, and it motivates me to educate on how to take care of our largest organ. Whatever you touch or walk on (with bare feet) gets absorbed into your body. It is of the highest importance to take care of our skin, from proper moisturization to exfoliation (and everything in between). Not to mention diet, exercise, water intake... we are just focusing on one small piece of the pie.
On a weekly basis your skin goes through a natural regeneration process. New skin cells are formed, and the dead skin cells pile up - if you have a build up of dead cells, your skin may be flaky, look dull, feel itchy, etc. Exfoliation helps to remove these dead skin cells so your skin will appear bright, glowy, and flake free. The standard rule of thumb is not to exfoliate more than three times a week - now, that doesn't mean three days in a row. BE GENTLE with your skin; the skin on your face is thinner than most areas of the body and requires gentle care.
Exfoliation is not created equally for each skin type. LISTEN TO YOUR SKIN. Sugar is a great exfoliant because of its fine grains. It provides a gentle exfoliation to rid your skin of the overlying layer of dead cells. At no time should exfoliation be painful, or cause damage to your skin.
My overall general advice to promote healthy skin is to wash your face daily, never go to bed without removing your makeup, provide nightly moisturization - I say nightly because you're not tempted to touch your face so the serum or lotion can really sink in - exfoliate a couple times a week to remove dead cells, and DRINK WATER.