It is a fashion now. Those girls and boys wearing the big black glasses without lens have no problem in the eye prescription. What they wear is just to help them look fashionable, cool and beautiful. Sometimes, they may wear the color contact lenses which will make their eyes big. And the outside glasses may make them look cool. This wearing is just an accessory.
You know that the big black glasses without lenses are a new born fashion here now. As you say, you see so many girls and boys wear big black glasses without lenses. Yes, this kind of phenomenon is ubiquitous because black is eternal classic style. Therefore, as a girl, I think it is a very good choice. But, you should also know that it all depends on different people, different flavors. Some people might think, black glasses are very cool because of dark color. On the other hand, there are also a lot of people don't like black, so they do not choose this kind of glasses. On the whole, this kind of glasses is very popular. Big black glasses can let girls look fashionable and trend, the same, and also can let a man more handsome and cool.
To be honest, this question used to hit my mind. I just wondered, is those people crazy or something? But later on, I realized that it is a trend that those big black frame glasses can make people look more erudite and cool, even if they have good vision. Also, by wearing the glasses, people can hide some of their facial defects, for example, a man with small eyes can make him look better in this way. Anyway, people have different tastes.
Big black glasses without lens are fashion items nowadays which are welcomed by most of young people and stars. They can flatter your face and make you outstanding in the crowd. There are various kinds, colors of theses glasses for you to choose. If you are a cool hunter, you must get at least two pairs of that glasses. You can wear them to attend classes, join parties, dating with friends, going shopping and so on. To sum up, big black glasses without lens are the bare necessities of fashion.