Who needs image descriptions or alt-text?

In general, everyone who cannot see the images in your post. They need something that can act as a replacement for the image.

This can have various causes, mostly either a disability or a technical limitation:

Disabilities

People who need an image description or an alt-text may

• be born blind
• have lost their eyesight at some point in their lives (most blind people have, by the way)
• be permanently visually impaired, i.e. they can see, but not well enough
• be temporarily visually impaired, i.e. they can't see well for a while
• have a cognitive impairment that makes it difficult for them to identify what the image shows

Technical limitations

People who need an image description due to technical limitations may

• have a bad and/or slow mobile Internet connection that doesn't let them see images
• have temporarily limited bandwidth because they've exceeded their monthly mobile data plan
• have turned off images on their mobile device to save bandwidth
• use a command-line-based, text-only browser or application (the Web browser Lynx, the Mastodon client Toot etc.) to access the Fediverse because they use their Linux or BSD exclusively on the command line, and they have no way to have images shown