You basically back up /home and maybe /etc/fstab if you’re using the same exact device with multiple drives but don’t want to set up your drive partitions again. If your device only has one physical drive, there’s no point in keeping /etc/fstab since the new distro installation would just create a new /etc/fstab anyways. Backing up /etc/fstab starts making sense if you have like a PC with 6+ physical drives. You could technically just set up all 6 drives through the installer, but I personally would never do that. Whenever I install an OS, I always physically disconnect every single drive except for the drive that the OS is going to. Otherwise, you run into the risk of installing the OS on the wrong drive, wiping data from that drive in the process.