In my opinion, concealer is meant to go over the areas foundation didn't cover. There are many different types of foundation, some with heavy duty coverage, some with medium coverage, in betweens, and then light coverages with tinted moisturizers. If foundation covers all you problem areas, then concealer is not necessarily required. Some individuals can even get away with just a touch of concealer and no foundation. Typically concealers are also in thicker more concentrated formulations therefore it's not something you want all over your face as it can get cakey. That's when I use a heavy duty foundation and then concealer for my left over blemishes.