Why alt is use in img tag?

Answer 1

alt means alternative...
If by chance coder forgets to attach image link or image link become corrupted then alternative text will be shown to screen letting the viewer that there is an img but link has some error.

Answer 2

Also for SEO

Answer 3

also used for accessibilty: screen readers will read this string to describe the picture ^^

Answer 4

Also browser use alt attribute for image searching.