If you need something on the internet, you would probably open your browser, go to Google and enter your search query. You scan through the results and go to the ones that you think are right. There's no rocket science involved here. However, imagine the same scenario for someone with disabilities.
Your site maybe be great- with really wonderful content, flashy graphics- and you might boast of a million users- but are you sure that each and every person who uses the internet can access all features of your site?