Every so often, I've run into a situation where being able to mark every chapter of a manga as read would be useful.
For instance:
Going back to a previous chapter in the past has led the app to think I want to read from that chapter onwards all over again; a way to reset this would be great
More than once I've started reading a manga on one site, only to find it didn't have the full run or the site disappeared mid-way through, meaning I had to start over elsewhere - and reading only the chapters I hadn't already left meant the app showed a large number of chapters as unread
today I accidentally clicked on "reset bookmarks" instead of "remove all chapters" when trying to clear out the downloaded chapters, and ended up having to manually scroll through ~90 chapters of a manga by holding the L button so the manga wasn't showing as unread in the library... only to find the bookmark had got stuck about 15 chapters in, so I had to restart the process!
Every so often, I've run into a situation where being able to mark every chapter of a manga as read would be useful.
For instance: