Una buena explicación en Alt vs Figcaption:
When we boil it all down, websites are really just a series of words and pictures. It’s a way for a person (or sometimes a robot, or sometimes a robot in a person trenchcoat) to tell you about a thing and you, a visitor to their internet page, to consume that thing in whatever form.
Sometimes that form is purely visual. An image.
For that we have the img element. Its sole requirement is the src attribute to direct your browser where to locate your image. And voilà! Visual splendor.
Images, however, have one more thing to consider: Assistive technologies need words to interpret an image. People sometimes need words to better understand why an image is being displayed.
Luckily we have a few ways to describe them in a human readable manner: alt and figcaption. These items house very different writing patterns though, and can be used both together and apart, depending on the conditions.