Because last time I installed MS-Windows 10 it had the most beautiful unlimited blinking cursor in the corner and nothing else. Until I installed GRUB from a GNU+Linux live CD whose root fs wasn't an actual device and that broke grub-probe. But fortunately grub-mkconfig is a shell script.

I don't know why it failed but it did and Grub saved the day.

In my opinion there is no reason for using GRUB for any purpose.

I have considered it obsolete and I have stopped using it more than a decade ago.

For the purpose of booting multiple operating systems on the same computer, including with multiple versions of the Linux kernel, I just keep a set of small USB memories, one for each operating system that I want to boot, which are formatted with FAT32 and which contain, in the case of Linux, the kernel, the initramfs file and syslinux to boot them.

This is much safer and much more foolproof than having a multiple boot configuration on the computer, which can be damaged by Windows at any time.

Even for people who do not want to boot from external memories, GRUB is not needed, because now multiple boot can be done from the UEFI menu, which can boot directly a Linux kernel without any extra boot manager.