Some podcasts remove all of their backlog when they "sell out" and go behind a paywall, having them backed up prevents that. (How did this get made?[0] being one example)
Also some podcasts (BBC ones I think) add ads while you download, based on your country. Some of my No Such Thing As A Fish[1] episodes have Christmas themed ads in them because that's when they we're cached :)
[0] https://www.earwolf.com/show/how-did-this-get-made/ [1] https://www.nosuchthingasafish.com
I use it as my podcast app.
I mostly only have the last 2-5 Episodes on my server for each Podcast. (you can automatically remove episodes if there are more than X)
Though I do keep all Episodes for 3 of my favourite Podcasts.
Or, as a couple that I've listened to and might re-listen later have, they later start injecting adverts where they were not previously present, or start piling more in where they were reasonable before.