Well, I can see that you are curious about why people wear sunglasses even if there is no strong sunshine. So, as a matter of fact, wearing sunglasses has become one of the most popular trends in the world. Sometimes people would wear sunglasses on a rainy day or under circumstances, because they appear to be awesome and have better looks, also hide their bad looks.
As we know, the sunglasses are mainly used to protect their eyes from the uv rays or other bad lights. They could protect the eyes to some degree because of the special coating part. However, many people now use the sunglasses for the use of making cool besides the practical use. That is why you see many people wearing the sunglasses to a day glow. They may want to show their coolness.