Images, charts, graphs, audio/video content, etc are considered as non-text content. People with certain disabilities will not be able to perceive the information conveyed through non-text content unless an alternate way is provided to convey the same.

For example, a visually challenged user may not understand if an image of phone is provided to represent a contact phone number. An alternate text need to be provided to convey the intention of the image.

People who use assistive technologies will be benefited from the alternate text of non-text content. For instance screen readers will be able to read the alternate text, users who have both hearing and visual difficulty can perceive the alternate text content via refreshable braille display. >br>
In case of complex images such as graphs, charts and audio video content a simple alternate text label can be provided to notify the user about the element and detailed description needs to be provided in a separate page or separate portion of the same page.