In earlier times, there was only one way to boot a PC-compatible computer. All of our desktops and servers had standard firmware called the "PC BIOS," all our hard drives used Master Boot Records at the beginning of the disk, where the PC BIOS would "look" to find boot loader code which would in turn load Linux, and our hard drives were partitioned into different regions using the standard MBR partition scheme. That was just how it was done. And we liked it that way!