Alternative Text and Accessibility
Per the Web Accessibility Initiative, "Images must have text alternatives that describe the information or function represented by them. This ensures that images can be used by people with various disabilities."
Alternative Text and Google Image Search Results
Filling in the alternative text field will also help with Google Search engine results. Per the Google Search Central, "By adding more context around images, results can become much more useful, which can lead to higher-quality traffic to your site. You can aid in the discovery process by making sure that your images and your site are optimized for Google Images."
The more descriptive the media item, the easier it'll be to find!