Improve on-page SEO and accessibility.
Based on your keywords research you should have assigned to the page you’re adding alt text to your images a main keyword and sometimes one or more secondary keywords. These are the keywords you want your page to rank for in search engines.
Search engine image recognition technology vastly improved and crawlers are able to interpret most images - if the image you are using on your page is unrelated to the topic of the page it’s possible you could either rank for unintended keywords or miss out on ranking altogether in image search results and your image optimization efforts will have no effect on general search results.
Search engines assess how an image relates to the surrounding text.
Check that image filenames follow file naming best practices and, where possible, include the targeted keyword in the file name.
Write a 100-125 character description for each image that includes the targeted keyword. Describe important aspects of the image for someone who can’t see it well, don’t stuff keywords, and avoid using phrases like image of and picture of.
Screen-reading tools typically stop reading alt text at the 100-125 character point.