Track down what the real use of alternate text is. It is important to not abuse it for SEO purposes or leverage it for marketing purposes. Let it be a concern of well-formedness rather than the bottom line.
Until one understands the need for accessibility, all the remaining explanations are meaningless. Why even use the
alt="" attribute? Just for show?
Alternate text is meant for people and user agents who/which literally cannot see. Blind people are blind. So too are search robots. Search engines are blind and use the data surrounding an image to give it some idea of how to index the image. The alternate text is the most specific since it is in the same tag as the src attribute. They must be connected is how a search engine will see it.
To a blind person, the alternate text does its best to describe the picture that the sighted user can see, in a brief phrase. The user will be able to browse the page and come across a descripton of an image then continue reading. If that description fits the context of what they are reading then it will be meaningfull and experiential. If not, well the experience could be less than pleasant and the page visit lost on the user. Accessibility means including everyone in the same relative experience.