At the core, making sure that materials created for students, faculty, and/or staff are accessible is about fairness--making sure everyone and anyone, no matter what their challenges might be, can have full access to and benefit from whatever the content is.
However, many people falsely assume that only people who are blind and/or deaf need accessible content--and some also assume that there are few (if any) disabled students/faculty/staff in their college or university. At points in the past, that belief might have been accurate, when people with disabilities either only went to schools with others like them (e.g. Gallaudet University for the deaf) or did not pursue higher education at all.
Advances in technology now mean that people with varying disabilities have more options for access to information, including in the realm of higher education, than ever before--but the technologies those people use (usually referred to as "assistive technology") can only take the person to a certain point if the content (books, documents, websites, videos) does not comply with established accessibility guidelines. Like all technology, assistive technology can only work with the information it is provided--so if a blind student would try to use a screen reader to access a print document, that would obviously not work because print content is not compatible with the reader.
Making content accessible at the beginning, during creation, is the easiest and most efficient way to not only have accessible content available but to be prepared for having a student/faculty/staff member who needs accessible content. It always takes more time and effort to convert non-accessible content, and for a student taking a course--time the student does not have access to the content could often affect their academic performance.
This page of the Accessibility Guide gives an introduction to the main types of disabilities that content creators need to be aware of, and it also includes a short list of the most basic ways content should be formatted to begin to address accessibility. Other pages of this Guide list more specific ways to make distinct types of content, including file types, into accessible content.
Although there are many different types of disabilities, the following 5 areas cover much of the disabilities of greatest impact to students/faculty/staff in higher education:
Visual disabilities include not only people who are blind (zero vision) but those with low vision (limited visual field, blind spots, blurry vision not correctable by glasses) and those with color blindness (inability to tell the difference between certain colors).
Hearing disabilities include people who are completely deaf as well as those with varying degrees of hearing loss.
Motor disabilities include not only people who have lost limbs (e.g. loss of hand(s)) but those with motor impairments that cause tremors, weakness, and/or paralysis.
Speech disabilities include people who are unable to speak at all as well as those with speech impediments.
Cognitive disabilities include not only people with learning disabilities (e.g. dyslexia) but also people with autism, ADHD, and/or other processing problems which could include those due to injury (e.g. brain injuries).
The list below includes some of the most basic, as well as some of the easiest to implement, concepts and formatting directions for making content accessible.