Online users with visual impairment and/or limited motor skills are legions. There are numerous reasons to make web content accessible to everyone, but: if only from a business perspective, it makes sense not to exclude these people, who are as potentially willing to spend their dollars with you as anyone else.
The variably legitimate excuse of cost for making buildings accessible to handicapped in the real world, this argument does not exist online: publishing accessible web content only requires a bit of experience, but mostly an awareness of the problems some disabled prospects face on the Internet. Nearly close your eyes, and read this text to simulate a vision impairment; semi-crutch your fingers and try to use your mouse to navigate most websites. Pretty hard, huh? These are two quick tricks that will help you empathize with "special-need" users, and guess what to do next.