There are 285 million people with visual impairments. If you have a website or web application that is not accessible then you are limiting your audience by at least 285 million people. It is a requirement that all government related websites meet section 508 accessibility guidelines, and your business website should as well. Accessibility is a right, not a privilege.
Common visual problems are due to cataracts, glaucoma, age-related macular degeneration, corneal opacity, diabetic retinopathy, childhood blindness, trachoma and onchocerclasis.