The key reason to plan website and other software accessibility is to enable all people to have equal access to tech. This slide show walks various personas through a popular HR recruiting site. It concentrates on the Assistive Technology devices they use.
A simple thing like using only a keyboard and not a mouse can make it difficult to navigate through a webpage.
If you are using a screen reader because you can’t see the screen and hit an error message which is silent, how do you know what just happened?
If page design is not logical is it more difficult to find what you want on a page when it is blown up to 800% of the designer’s expected size?