Imagine you are a freshman student in your first semester at college. You have been blind your entire life, but have learned to deal with the aspects of life that most others take for granted. You can read Braille and are quite comfortable in using a computer with a screen reader that speaks aloud everything that a sighted person would read on a screen.
You are taking a very enjoyable course in your major, and were able to get your textbook in Braille without any trouble or extra cost. You have been keeping up with the material until the fourth week, when the instructor asks everyone to review a video and several instructional documents before an important class session. You go online and access the video, but you can tell something's missing. You can hear the audio track, but the speaker is referencing information shown in the video that you can't see, and the speaker isn't reading that information aloud. So, you check the documents to find out what is in them, but your screen reader indicates that they have a lot of images and very little text. When you finally ask your roommate to tell you what is in those documents, he says they have a lot of charts and diagrams along with scanned images of hand written notes, none of which you can use since you can't see them.
There is one document that doesn't have any images, so you focus on it, hoping you can get some idea of the material being covered in your next class. You listen to the entire page read by your screen reader, but you need to go back and review couple of sections. However, when you ask your screen reader for navigational cues, there are none, because none of the section headings were labeled as such. So, your only choice is to listen to the entire page over again, and even worse, several sections have long lists of references that you don't need to hear again right now, but you can't easily skip past them because they weren't labeled as lists, so your screen reader doesn't know where a list ends and the next paragraph of regular text begins. You finally re-hear the sections that you needed to study, but you spend twice as much time as you should have, you still don't know what was in the video and other documents, and all of that extra effort keeps you from finishing one of your other homework assignments.