Men's running shirts are engineered specifically to make running better.
While cotton is durable and soft, it's not good for sweating. Cotton soaks up sweat and moisture, but dries slowly, which means it'll be clinging to your skin—or worse: chafing—for the rest of your run.
Many new running shirts are made from technical fabrics, like polyester, that are soft, lightweight and encourage evaporation. Other running shirts for men use fabric blends that can add a bit of stretch for a better fit, and some are made from soft merino wool that wicks moistures and is naturally odor-resistant.
You can also shop for men's running shirts with an athletic cut. Like wearing cotton, a baggy running shirt will catch wind or flop around during your run, which can be annoying and slow you down. Buying a shirt with a trimmer fit can help keep excess fabric out of the way.
Many brands also sell men's running shirts with details such as reflective designs to keep you visible in low-light situations, smooth seams to mitigate chafing, and short or long sleeve running shirts to give you the right length for any weather.