Imagine your entire experience of the Web is through a screen reader.
You're “reading” an article, and you're very interested in the topic. Then the author decides a picture is worth a thousand words, and they provide a useful chart or diagram:
The article then proceeds to draw conclusions based on the graphic:
As you see, not all audiences are able to process information graphics with the same degree of success.
But you don't see, because you couldn't see the graphic, and the author didn't describe it adequately in the alt text. Hey, at least this author was thoughtful enough to provide alt text... many authors don't even do that!
You see the problem.
Now, show of hands: how many of you want me to do this whole presentation using a screen reader?