Excerpt below: 

"Today is Global Accessibility Awareness Day (GAAD), a time to celebrate and explore the many ways users and publishers can make the Internet more accessible to people with disabilities.

Image alt text is a key element of web accessibility. About 7 million people in the United States have a visual disability, which can make it challenging to navigate an Internet that has become increasingly driven by graphics and videos. Many of these users access websites via screen readers, special software programs which transform webpages into audio, allowing visually impaired users to navigate and interact with them.

Screen reader technology has improved dramatically as the Internet has matured, and many modern web standards increase usability dramatically for visually impaired website visitors. But in many cases, screen readers still have a literal blind spot: images."