It's not about the internal storage. U can back that up manually, the twrp backup is a way to restore the data that's not in the internal storage. It's basically a way to restore a ROM/OOS like you never left or what it was like before the backup. It's way easier to switch from a twrp backup if you got loads of data/ stuff to set up. Otherwise, clean installing is better. For me, switching ROMs is tiering and a long process to get everything set up and restored.I don't understand why people need the backup function that badly. The only time your internal storage is fully wiped with twrp, is if you're going from OOS to custom or custom to OOS. Other than that, you can dirty flash custom ROMs(keeps installed apps and rom settings)or clean flash(factory reset) both of which keeps all your media like music, videos, downloads, etc.
TWRP backup is just something to have in case u want to switch to another ROM and switch back to or if a mod ur going to install might mess the system up or something to have in case something goes wrong. If any of these things do happen, a TWRP backup is handy especially when u just want to go back to a ROM quickly + easily with everything back and running
Ps. I am coming from a OP6T and that's what the TWRP backup was mainly used for. Idk know if this is the same for the 8 series