To provide more to the "why", although end-user devices have moved away from BIOS-only boot, there are large number of systems that have no alternative to BIOS, and the hardware cannot be upgraded, only the firmware and software.
Most of the systems I have ran into were in SCADA[0], PMS[1], and BMS[2].
For BIOS boot, the BIOS looks at the first couple of blocks on a hard drive for the boot code. This is the first GPT partition that gets created, and the future grub-install code injects the BIOS bootloader there. Thus, to support BIOS and UEFI, you need the BIOS bootloader at the beginning of the drive.
Yes, but why would you want the BIOS boot assuming you have a motherboard made in last 10 years and it has a UEFI implementation which isn't completely broken.
I might understand doing that just in case when preparing a bootable flash drive. Why complicate things for permanent installations where you know your current hardware and your next system after 5 years is unlikely to much worse than current one?
Why do you still need the legacy BIOS boot logic?