Study and research support the idea that putting on face masks in public, even when you feel well, can help curb the spread of the coronavirus — since people can spread the virus even without manifesting symptoms. That's the sole reason to wear a mask, and that is to protect other people from you.
Cotton face masks can also offer the wearer some protection — even if the cost varies greatly, depending on the type of mask. Not one mask will offer full protection, and they must not be seen as an alternative for physical distancing from others, constant hand-washing and staying away from crowds. The moment you combine masks with those measures, they can make a huge difference.