Why Accessibility?

There are many reasons you should make your website accessible:

It's the right thing to do
Imagine the new opportunities the web has created for people with sight problems. Then imagine how frustrating it is to find a website that you can't actually use.
It's easy to do
You don't have to do it all at once. You can change aspects of your website one bit at a time.
You have a legal obligation
You must make adjustments to your website if it is unreasonably difficult or impossible for disabled people to access.
You're reaching more people
Visual impairment isn't the only disability you should cater for, but remember that there are 2 million people with sight problems in the UK
You will be able to meet other standards more easily

For example, charities which provide health information may want to apply to be accredited as NHS Information Partners. Part of the accreditation requires you to have an accessible website.
It's good for Google
Many accessibility features, such as descriptive alt attributes for images, and title attributes for links, will help you get a higher ranking in Google