The plugin adds an accessibility issues filter to the media library which allows you to quickly identify images that are missing or invalid alternate text.
The fact that images are missing alternate text while in the Media Library may not be a problem but many websites are designed to dynamically place images on parts of your website and when not entered in the Media Library can result in issues on your website. When using the deep scan feature described in the Plugin Settings section below these issues will be identified but this feature is provided as another means to improve the overall accessibility of your website and can be used as required.