Accessibility. 

Some people who use Mastodon are visually impaired and use screen readers. Some people who use Mastodon have images turned off to conserve their data usage. Sometimes disk errors or server errors happen, or your admin decides to prune old files, and your image just doesn't load anymore. Captioning an image allows people in these conditions to participate in your toots with full context.

You can also use image captions to insert additional jokes (like webcomics often do) or additional comme