First and foremost, cotton is by far one of the most comfortable fabrics available. It's gentle on the skin. Most people will agree that cotton is never itchy and feels fresh all year long. Obviously, there are many different blends and types of cotton available, each differing in quality and comfort. Cotton generally offers superior comfort.
Cotton breathes better than most fabrics and offers great absorbency.
Just think of the clothing you wear to sleep or when you go to the gym. I can bet that they are mostly made of cotton. These examples support my last remark that cotton t-shirts or sweaters allow for easy breathing and absorb excess sweat. The same principle applies for trendier articles of clothing such as shirts and trendier t-shirts. The idea is not simply choosing cotton-made apparel, but if you have the choice between two equally trendy sweaters and one is made from synthetic fibers while the other from cotton, opt for the cotton one for its comfort, absorbency and breathability.