Changing root is commonly done for performing system maintenance on systems where booting and/or logging in is no longer possible. Common examples are:
- Reinstalling the bootloader.
- Rebuilding the initramfs image.
- Upgrading or downgrading packages.
- Resetting a forgotten password.
- or just when you broke your system and can no longer boot.
Then we mount the partitions we need and undo whatever broke your system.
mount /dev/sda3 /mnt
mount /dev/sda1 /mnt/boot