From the earliest days I put back there (as well as after \endinput) bits and pieces I don't want to discard, but don't make it to the final draft of the paper either. This way I don't clutter my directories with zettel, and I can find the discarded material for other purposes later. Then some people want opendoc/openxml copies for whatever reason, and then I found that the supposedly ignored materials pop up in the converted files.

BTW I also miss the ability to comment out material with the {comment} environment: but then regular %s do the job. 

Besides, the inability to handle materials this way with MarkDown is what keeps me from moving to that format altogether.

Cheers,