Primers and Concealers are completely different things. Depending on your skin condition or the look that you want to achieve, you may need either one or both.
So let’s look at each one of these in detail:
Primers - A primer is used to prep your face and to get it ready for the application of foundation and other makeup. They fill in fine lines, wrinkles and any unevenness on your skin. A lot of people skip primer in their makeup routine but I wouldn't recommend it.
Using primers the right way can actually make the application of foundation and concealers easier. This is because primers actually help your foundation and other makeup "stick on" better.
Many primers on the market today also have skin protection and nourishing properties that make them worthwhile to consider in your makeup routine.
If you're not sure of how to apply primer, I have written a complete guide on this subject here.
Concealers - Concealers are designed to cover tough spots that your foundation couldn't cover. Some people have severe skin conditions that are challenging to cover even for full coverage foundations.
Concealers usually have a thicker formula than foundation and it's commonly applied only on areas that require additional coverage.
Concealers usually come in less shade options than your foundations. Therefore, it's recommended to apply concealers before foundation.