Why make a website or web application accessible?

Lately, a couple of people have asked me why is accessibility important? Below is a section from a documentation I am currently writing on web accessibility.

Why make a web application accessible?
The web plays a vital role in everyday life allowing us to access information for education, employment, government, health and social care, recreation, and more. Today, up to 20% of website users suffer from some form of the disabilities mentioned earlier. This statistic does not include ageing – something that we all suffer from and subsequently increases difficulty levels when using the web.

The Disability Rights Commission found that there are 8.5million people in Britain using the web who suffers from a disability. Combined they have the spending power of £40 billion. Regardless of this fact, making websites accessible is still not considered worthwhile by designers, developers and even creative managers.

Web accessibility does not only stop at those with physical or mental disabilities. Many users do not have access to the latest WebKit browsers or mobile devices. As of April 2011, W3C schools report that more than 8% of users still use Internet Explorer 7 or Internet Explorer 6 while 41% of users use either Microsoft Windows XP or Microsoft Windows 2003. It is a fair assumption, based on these statistics that the majority of these users may still use a dial up connection or a broadband speed of less than 1mb. Some of these users will only have access to browsers that do not display images while others may only have access to browsers that do not support Cascading Style Sheets.

When a website or web application is made accessible it expands the audience to include all of these users in addition to those suffering from mental or physical disabilities.