The alt tag is meant to show alternative text in lieu of being able to show an image. While images might look great visually, they are useless to blind people or people using text-based web browsers. Therefore, the alt text is used to describe the image if is unable to be displayed, for accessibility purposes.
As the name is mention the alt tag means the alternative text so it is to be in the ears that the search enginges cannot read the pictures so to make them clear we have to use the alt tags to make them clear what we have in pictures .
Alt tag is benefitial for website as per the seo view. ALT tag is stand for alternate text like if you having image on website then when you point a mouse cursor on image it will highlighting the text if ALT tag is tagged in image. You can target your keyword in alt tag which makes a good impact to rank well in google SERP.
alt tags are the tags used for image optimization..
as google while crawling a site crwals the images too so it is neccesay to inser a alt tag as then only a image can be optimized and if the post image is optimized traffic increases which in turn helps in improving pr and alexa ranking..!!