You do your makeup differently throughout the year as you would switch up your skincare routine throughout the year. During the summer some people prefer a lightweight makeup look, whereas others just keep their same routine. Foundation is better to use during the winter because it protects your skin during the cold. Using only concealer is better during the summer because sweat is less likely to mess up your look.
Using only concealer for your makeup limits the chances of your pores being clogged compared to using foundation. Overall, you get a cleaner look that is also more time manageable. Using foundation evens out your entire skin tone, but makes finding the right shade match more difficult with concealer. This type of makeup is better for special events because it gives you a polished and long-lasting look.
Use concealer for dark spots on your face like directly under your eyes and the high points of your face (for a more lifted look). Using concealer on these areas brighten those spots and clean up your skin. Foundation is meant to be used on your entire face. To find out the right shade, apply it on a small area of your face in natural light, if it doesn't disappear then it is not the right shade.
In terms of skin health, concealer is better for a variety of reasons, but primarily because there is less product buildup in comparison to using foundation. Also, dirty makeup brushes can cause breakouts just as too much product can.