After changing breakpoint sizes for uploaded images or installing a plugin that modifies media files, you have to upload all images again so that they get processed. Not a big deal if you don't care about alt-text or caption, but if you do, it's not fun to enter that stuff all over again.

So it would be nice if there would be a button in the media library for reprocessing all images.

The original image stays untouched, but every processed image will be deleted and processed again.