Why do you need alt text on your images?

The purpose of alt attributes is to provide a description of the contents of an image file. One of the most cited uses of alt attributes is to provide text for visitors who can't see images in their browsers. ... If a viewer can see your images, alt attributes will also show when the user hovers over your image.

For search engine  to better understand the context of the page and for the visually impaired to understand what image was added.

The most cited uses of the alt attribute is to provide text for visitors who cannot see the image in their browser. ... If a viewer can see your images, Alt properties will also be shown when the user hovers over your image.