Architects need to be concerned about the accessibility of their buildings in order to accommodate mobility, visual and other impairments since these buildings are open to all people. So it goes for web sites. A site on the web is open for all to see. Making a web site accessible to all is actually a very simple thing to do. World Wide Web Consortium's Web Accessibility Initiative is a good place to visit to understand more about how visually impaired folks view the web.
Mobility impairment is another thing that is usually overlooked.
A little common courtesy goes a long way in making the web a better place for all of us.