Many people want to keep the source files forever
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As Harry said many conversions aren't successful and often you don't notice the problems until attempting to read the book. And/or you want to change options, and/or calibre improves its outputs.
Of course you can always get you original from your seperate folder but this is a pain if you wanted to reconvert 1000 books.
For the dropbox issue or backing up you can easily bulk remove unwanted formats.
select the format you most commonly convert to and right click and select remove all formats but option.
Helen