Becoming Web Accessible has many benefits including compliancy for: mobile phones, Web-TV, kiosks, low bandwidth, noisy environments (difficult to hear the audio), screen-glare (difficult to see the screen), driving (eyes and hands are "busy"), redundant text/audio/video can support different learning styles, style sheets can support more efficient page transmission and site maintenance, captioning of audio files supports better machine indexing of content, and faster searching of content.
10% to 20% of the world's population have some form of a disability. Not all of those disabilities affect access to the Web, however some disibilities do. For example disabilities with vision, hearing, dexterity, short-term memory problems. However disabilities such as difficulty walking, heart condition, etc., don't affect access.
Other issues are the ages of the web users. The average age of population in many countries is increasing, and aging sometimes results in combinations of accessibility issues: vision & hearing changes, changes in dexterity & memory. Few organizations can afford to deliberately miss this market sector.
Besides, a number of governments (including the US) require Web accessibility for certain kinds of sites.