i wear glasses sometimes because one of my eyes is slightly worse than my other (one of mine is basically perfect, the other is just a little bit off perfect. nothing noticeable), and so my brain has to work more to interpret what i'm seeing (it has to dull down the perfect image to the not-so-perfect image so that i don't go insane or something).
as a result i get awful migraines sometimes usually after reading something on a really bright screen or a book in the sunlight or something. also tests are awful so i wear them then. basically any time i'm doing a bunch of reading.