Alt text is used to describe images, graphs and other media used in presentations, websites, and other documents. Blind and low vision users require alt text to understand what an image is showing them through their assistive technology software. Alt text should accurately describe the image so the user can understand all the crucial information provided in the image. Screen readers cannot read images, graphs, etc, unless alt text is provided. Without alt text, this creates a barrier to learning.
Alt text is not only beneficial to those with low vision, blind, but also can be helpful for certain cognitive disabilities as well. Alt text can be helpful in other ways too. For example, if an image isn't loading on a website a user can still determine what image is of by the alt text. It also allows search engines to find the images in their searches.
Decorative images such as clip art or a graphic which is used for decorative purposes, do not need alt text. They are ignored by assistive technologies and are not relevant for the user.