If an image does not have an Alt Tag, then a visually impaired user viewing the document will not know what the image is. The screen reader will read [BLANK].
An ALT TAG is read by screen readers in place of images allowing the content and function of the image to be accessible to those with visual or certain cognitive disabilities.
An ALT TAG is displayed in place of the image in browsers if the image file is not loaded or when the user has chosen not to view images.
An ALT TAG provides a semantic meaning and description to images which can be read by search engines or be used to later determine the content of the image from page context alone.