Why is text so central to accessibility? Because text is highly interoperable. That is, systems of letters can be translated into code points and interpreted by all sorts of different software. Oh and humans understand text already, of course.
Because different machines can all read and write text, information can be interpreted and conveyed to humans in different ways. Primarily visually (in letter forms) but also aurally (as synthesized speech) and even by touch (refreshable Braille displays, for instance).UX accessibility with aria-label by Heydon Pickering