Why is alt text important in Excel?

Alt text helps people who can’t see the screen to understand what’s important in images and other visuals. Screen readers can let the user know that there is a graphic in the spreadsheet, however, they can not tell the user what it is, or interpret the content. Alt text adds a descriptive label that screen readers can read.
Visual content includes pictures, clip art, SmartArt graphics, shapes, groups, charts, embedded objects, ink, and videos.