iTunes can stream Podcasts without making a local copy, or you can download a copy to play as you choose, even if the original file is removed from the feed. Typically podcasts from BBC comedy shows are only available for a month for example, so I like to download those. Generally if you delete an entry from the library you will be prompted to remove the source file from the media folder, but you don't have to, so potentially your media folder might contain some files that you've removed from the library and no longer want, but they could equally be files you do want after all. You can add the media folder to the library to reimport anything not currently known to the library, then delete and trash anything you want to throw away.
tt2