Now a lot of people like to wear the glasses to improve their whole looks. They could choose to wear the glasses to match with their whole look. I think the reason why many Japanese people tend to wear glasses is for the look. Sometimes wearing glasses will make them look fashionable and modern. And the glasses could be with no prescription.
I believe that Asian people have the worst eyesight all over the world, especially in East Asian. Japanese along with Chinese has a high rate of myopia. So there is no wonder you see so many Japanese wearing eyeglasses. I have many Chinese friends, and many of them wearing eye glasses or contact lenses. The reason way there are so many people wear eye glasses in these countries is simple: young students spend a lot of time on study and reading. They get myopia or other eye condition at a very early age. Unlike western kids, these kids spend less time on outside door activities.
Not only Japanese, people at large may use glasses if required, Some of them for style others for actually seeing. Reason why they always wear glasses is because glasses make their life more convenient.