It is easy to understand. wearing dark eyeglasses looks cool especially for men. Maybe you don't think so, but there are some people fantastic to dark eyeglasses. Look some famous movies like men in black, they are really cool with dark eyeglasses. Besides, wearing dark eyeglasses can protect eyes in sun. With it, they can block UV rays, reduce strong lights, thus to provide you comfortable vision.
Dark eyeglasses usually have large frames that can cover a large part of the face, thus the eyes and face features under the dark glasses are invisible. Dark eyeglasses can also be called sunglasses. People wear sunglasses mostly for UV protection because their eyes are easy to get damaged if they are exposed to UV rays directly. Some people like the blind wear dark eyeglasses to hide their eyes which may be injured from being seen by other people.
What you refer to as dark eyeglasses are actually called sunglasses. People wear them mainly to avoid the deadly UV rays in the sunlight. Also, there is a fashion related reason that could explain people's donning them and that has to do with the sense of coolness those glasses provide. Men in particular like wearing sunglasses to enhance their images and attract women sometimes.