Why do you need alt text on your images?

It is nothing but Title of the image and should be related to the image, but not your keyword.

Because Google not read your image. Google only crawl alt tag of your image. If you search image of anything Google show results which have alt tag related your search query..

You are using the alt text to your image because through alt text search engine crawlers are actually know that what image is related to. and your image display in the image search when some one search about  the entered alt text.