I use it.
It populates all the thumbnails so that as you navigate, there is no lag time as these need to be generated. Periodically I will run the tool and it will generate the missing thumbnails. To see this lag, go to your media list with the gallery tab selected in the bottom bar then use the up or down arrow to navigate the list.
And I always run it after making major changes to my media file/folder structure. When I do this, I delete the existing thumbnails stored in the user files and then run the tool to repopulate the thumbnails.
It used to take many hours (I ran it overnight for 21K media files) after a major change. It now takes an hour to do a complete rebuild. Obviously fewer files equals less time. Doing it as an ongoing maintenance is much faster as it only does the missing thumbnails.
This will generate the new thumbnails for cropped of better saved images as Hagen experiences but does not solve the checksum issue.
And no, it does not flush the thumbnails before a rebuild.