One of the Biggest reasons to make sure your web site is accessible is that in a lot of cases a certain level of accessibility is required and if your site does not meet these requirments you could be fined etc.
It would be impossible for me write an article that covers the legal requirements in terms of accessibility so instead I am going to post a few a key links below for you to do your own research.
USA
Canada
- WEBAIM – World Laws Canada
- Common Look and Feel Standards for the Internet, Part 2: Standard on the Accessibility, Interoperability and Usability of Web Sites
Australia
- Australian Human Rights Commission World Wide Web Access: Disability Discrimination Act Advisory Notes
- W3C Australia: State Policies Relating To Web Accessibility
New Zealand