More than the disabled, accessibility goes beyond just designing for people with physical or cognitive disabilities. Accessibility is about making your site available to the widest possible audience no matter what their browser, device, connection speed or disability.
Accessibility makes sense from a business perspective as well as a moral and legal one. Why turn away visitors to your site just because they are using a dial up connection, do not have the latest browser technology or have poor vision?
Even meeting the most basic levels of accessible design puts you ahead of the majority of sites online.