The same photo could appear on three different pages and require different alt text for each instance. The alt text for an image can vary, depending on the context of the image, its role on the page, and the intended message.

For people who don't use screen readers, it can be difficult to understand how screen readers work. If you're interested in learning how people use screen readers, check out this demo from Marc Sutton and The University of California San Francisco.

https://youtu.be/dEbl5jvLKGQ