I love the mastodon image captioning feature because there are a lot of good uses even for sighted users
- when federation / the other instance / your router fails & the image won't load
- explaining visual jokes while "keeping the medium intact"
- naming a visual symbol you /think/ people will know about but maybe not, so they can look it up & read more about it
- noting 'what to look for' when it's unclear
- translating an image in another language while still letting it speak for itself