1. It helps with search engine optimisation.
2. It helps with assistive technologies if readers are vision impaired.
3. The spiders which crawl the web for information cannot see pictures. They read text. I don’t know if that is the case with real spiders!
4. If the web server cannot publish or show the pictures for any reason your text will still be there to alert readers to the fact you have an image.
5. You text might not be found but the alt text might be found and bring readers to your post.
Alt text is a safeguard and an optimization strategy.