In the digital days, creating documents that can be digested by everyone is important. Think about it: people with low or no vision may have the document "read" to them by screen reader software. So to have descriptions about images or graphics is important. That's just one of the steps for creating accessible documents.
Watch the video below, and...
….to learn more about accessibility in Office, check out the corresponding article at Office.com.
[Via: Office blog]