This description appears if, for some reason, the image cannot be displayed. The ability of search engines to recognize the content of images is still limited, so they rely on alternative descriptions to determine what the image contains. The alt attribute can influence the content for which search engines consider a website to be relevant. Ideally, the alternative descriptions should also contain the search terms for which the website is optimized.
The use of images will make your website more attractive and can help keep visitors on it. But what is also important, additional traffic can come to your website from image searches.
It is recommended to include both the keyword describing the topic of the page and an accurate description of the content depicted on the image in the Alt Tag.