What Blogger labels as the "Post Feed Redirect URL" would be, more accurately, labeled as the "Post Feed Subscription Redirect URL". or possibly the "Redirected Post Feed URL". Use of this setting does not redirect the feed to another URL, it simply redirects references to the feed (for instance, subscriptions by your readers) to the URL of another feed, which is sourced from your blog feed.
The most famous (though not the only possibility) redirection choice is FeedBurner. By "burning" a FeedBurner feed based upon your blog post feed, you can give your readers many choices that are not provided in the native blog posts feed.
Blogger instructs us
If you have burned your post feed with FeedBurner, or used another service to process your feed, enter the full feed URL here. Blogger will redirect all post feed traffic to this address. Leave this blank for no redirection.It's possible, however, that not all blog owners read this instruction, and simply interpret "Post Feed Redirect URL" as a setting which automatically changes the URL of the feed. And that's where some babble starts.