My non-scientific explanation for why it helps: After a pimple, the affected skin is very sensitive to the sun’s rays and gets “tanned” more than the rest of the skin leaving a dark spot. The more pimples you get the more dark spots you’ll end up with. The sunblock blocks those rays and minimizes their impact. [Below is my skin with no make up.]
Other benefits to clearer skin — Because my dark spots are a lot more subtle it minimize how much make up I have to put on and allows me to use lighter coverage foundation. I still wear my Mac Fix Fluid foundation (which is medium to full coverage) but since my skin has evened out, I can dilute it with my moisturizer and still get good results. I mix one pump of foundation and 1-2 pumps of moisturizer to create a home made tinted moisturizer. I started doing this last summer when I noticed my skin had cleared up and I’ve been doing it ever since.