<podcast:guid>
A permanent identity for your show
A feed GUID is a permanent identifier that belongs to the podcast rather than to its current hosting company or its current web address. Podcasts move hosts, get renamed and change feed URLs constantly, and historically every one of those events quietly broke listeners' subscriptions. A stable GUID means an app can follow a show through all of it, and means your subscription list can be carried between apps as a list of shows rather than a list of fragile URLs.
What you get as a listener
Your follows survive a show moving hosts, changing its title, or switching its feed address.
What the show does
One stable identifier that belongs to the podcast rather than to whoever is hosting it this year.
In the app
Resume positions — where you stopped stays with the episode