This is similar to Also use alt tag as title on images, which asked to make the alt-text serve as the title. However, the OP in that question could not think of a good use case.
I can think of a very good use case, having just gone through the tedious process of adding alt-text to my answer on why the Earth has tidal bulges on both sides. The use case is that what I wrote as alt-text was utterly invisible to me in the rendered result.
This is a problem. I oftentimes only see my typos, my putting "the" on the end of one line and "the" on the start of the next, my incomplete sentences, etc., when I read the rendered result. Because the alt-text is completely invisible to me, I know that it contains garbage. Making the alt-text serve as the title, at least for the author in preview mode, will help those who bother to use alt-text improve the accessibility of their answers.
My feature request is simple: In preview mode, make the alt-text the title in the rendering if the title of an image is missing and the alt-text is present.