It will protect your face and eyes from the sun, and shade your whole body. Better hats are ventilated, and any hat can be removed while you are in the shade and used as a fan. The best all-around travel shirts are long sleeved, button down, and large in size. These will keep you shaded, and provide good ventilation, especially with the arms rolled up. They are nylon and dry much faster, important in hot, humid climates.
Tee shirts are also very comfortable, and are lighter to carry.
Again, large sizing is important. Having a sweaty shirt clinging to you is uncomfortable and doesn t allow air to circulate like it should. Avoid large designs and silk screens on the shirts that prevent the fabric from breathing. The North Face make nylon tek tees that dry in minutes and are very light. I brought three along on my latest middle east escapades and they served very well.