Why must "device for bootloader" be
/dev/sda instead of
Is it that GRUB can't be installed on a partition?
How does BIOS recognize the bootable devices? Is it that installation writes into BIOS?
In BIOS mode, the computer system looks for a bootloader in the very head end of the computer, which is pointed to by
/dev/sdxn points to the head end of partition n in drive x.
If you have a bootloader in very head end of the computer (MBR), you can chainload to the head end of a partition (PBR), and in this case it can be useful to install a (second) bootloader there.
If you do not want to overwrite the head end of a partition (MBR), you can write to the head end of partition (PBR) without chainloading. Later on you can run
sudo update-grub in the main operating system and get grub menuentries for the new (dual- or multi-boot) operating system.