Answer 1

alt - alternative
Imagine that you have some 'img' tag with some link to an image. If the image doesn't exist (the link to it is broken), then you can put information about what is on the image, so instead the image you'll have that information.
But the most important thing is that the screen readers reads the content of the alt attribute to help to the visually impaired users to know what is on the image.
So it's best practice always to use the alt attribute.