What's the purpose of fetch mode, as opposed to streaming mode? Is it a legacy of bygone days that just hasn't been deprecated, or is there something I don't understand from reading https://www.postgresql.org/docs/14/app-pgbasebackup.html?
Your backup can go to a single file with it, which it can't do in streaming. Which means it can also be sent through a pipe.
It also needs one connection instead of two to the server, if that's limited.