Example: I am running out of space on my phone. I look, and see Signal is taking 4gb of space. But I would like to preserve a lot of that media. So I buy a mini-SD card and install it in my phone. Yay! Now to turn on Signal chat backups!
Oh, but the backups are hardcoded to a location on your primary storage that you can't change. so even though I have 126gb of free space on the SD card I installed, I can't back Signal up to it and I am still running out of space.
My only option is to try removing every other app I have installed, and hope there is enough space reclaimed to perform a backup, which I can then copy off my system so I can reinstall my apps...WTF.
Meanwhile there is no way to back up a single chat. You can archive it, but that just removes it from your Signal home screen, it doesn't actually create an archive of any kind. And there is no way to back anything up on desktop.
This isn't a feature, it's an antipattern.