Final answer:
The author includes captions under images to provide context and relevant information that complements the visual content, enhancing the narrative and helping the reader understand the connection between images in the context of the document.
Step-by-step explanation:
The best explanation for why an author includes a caption under an image is to provide context and any other important information that cannot be gathered simply by looking at the image. The purpose of captions is to enhance the narrative by adding a textual element that complements the visual information. Such captions are strategically placed to guide readers through a narrative, to make links between images more clear, as well as serve an organizational function by breaking up the text and focusing attention on the image itself.