-Reading public domain books: gutenberg offers books in epub format.
-Fanfic: archiveofourown.org also offers fic downloads as plain epubs
-Scanning my own physical books to make a backup copy: this is slow and tedious in the extreme, so I don't do it much, but I generally save such books as epubs unless the formatting is really complicated, in which case they get saved as ordinary pdfs.
-Research papers: usually pdf format.
Until the major publishers start selling books that I want without DRM, those four categories make up all of the books on my device. That's why I've never used kepubs.