It is a bit confusing to reconcile the different meanings of grub 'installation'. On the one hand, it is a bootloader that is placed into the MBR or boot record of a partition. The other meaning of 'installing grub' is just like installing most other software. It is a collection of applications that can be run as necessary. One of the things that grub, the application, can do is to install grub, the bootloader. In the latter sense, one can install grub, and use it to place a grub bootloader on a target disk. This can be done without affecting the bootloader of the build (non-target) host.