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.
A concealer is for under eye circles or to correct colour problems on your face. Some concealers come in different colours such as green which helps you cover zits and spots.
A foundation is for smoothing out the texture of your face. It is also for evening out your skin tone on your face. It cannot conceal as well as a concealer. However, some might use foundation as a concealer, it is up to you.
It depends on how much coverage you need and what kind of foundation you use. I prefer mineral powder foundation to liquid makeup and don't need a ton of coverage overall, but without a nice thick concealer under my eyes I look like I haven't slept in a week.
If your foundation covers your undereye area enough for you, then there is no reason for you to buy concealer.