What is the function of <alt=""> and why it is used?

Answer 1

The required alt attribute specifies an alternate text for an image, if the image cannot be displayed.

Answer 2

The purpose of the alt is to have backup text in case the image don't load or is not supported by the browser. Also if someone that is blind is using a text to speech program, the program will read the alt because obviously it can't read the image.