Why Should I Rebuild Thumbnails?
If you have a website and want to change WordPress theme, whether your previous theme supports thumbnail or not, new theme will use main image files (even if thumbnails exist, new theme probably cannot use them). That makes your website heavy and sluggish (specially in mobile devices) and uses server bandwidth. Uploading every image again in most cases is impossible. But a built-in tool in Publisher makes it possible to create new thumbnails according to Publisher demand.
Rebuild Thumbnails in Publisher Theme?
If you are changing your theme to Publisher and there are images on your website, use theme built-in feature Images Regenerator in BetterStudio → Rebuild Thumbnails to regenerate all your thumbnails for better performance and image quality on your site.
Also there is a checkbox for rebuilding all images or just featured images.