When wouldn't you want to install GRUB bootloader?

When would you not want to install the GRUB bootloader?

Possible reasons:

• You plan to boot Linux using the bootloader of another installed OS, such as Windows.
• You plan to boot the Linux you're currently installing using the grub from another Linux installation on the same machine.
• You want to use GRUB legacy when the installer is pushing GRUB2 (or vice versa).
• You want to use LILO instead of GRUB for great justice.
• You want to use SYSLINUX instead of GRUB just to make life hard.
• You plan to boot only from write-once optical media for security. << This is why I don't install grub.

On an UEFI system (most modern PCs these days) you skip all the complexity that GRUB brings. GRUB has two functions: it serves as a boot manager, i.e. it shows a menu of entries that you can boot, and it acts as a Linux bootloader. In addition to this it knows how to read from a variety of file systems, and it can chainload another boot loader, typically for Windows.

On a UEFI system you can do without all this. The Linux kernel knows how to load itself (if the kernel has been configured to include the EFI Stub Loader), so the boot loading part of GRUB is redundant. The UEFI firmware ("BIOS") can load the kernel, and the kernel can set itself up in memory and start running. The firmware also contains a boot manager, but you can install an alternative simple boot manager like systemd-boot.

In short: there is simply no need for GRUB on a modern system.