Bios and Bootloader are 2 differents things.
Your bios (Basic Input Output System) is basically a primitive level. He can detect disks, and performs somes really basic tasks on the hardware.
Your bios will detect the drives connected to your motherboard, and then access to the MBR (Master Boot Record).
On this MBR you will find grub (or another bootloader), generally with an access to /boot with the kernels and minios and it 'detect' also the others systems availables.
Then the bootloader load vmlinuz, initsystem (systemd or sysinitV), kernel...etc
Thats why you have to use grub anyway. Your bios cant see any os, and wouldn't know what to do with it.
Even Windows use a bootloader.. its just hidden, but you might see it if you install 2 differents version of the Microsoft OS
