You don't if all you want to do is change your
grub.cfg. You can just mount the
chroot to it and make the changes you need.
You do need to mount the other directories if you want
grub to autodetect any other installed operating systems since it needs access to those directories to be able to properly probe the system. By mounting them under your
chroot environment, you mimic your installed system and trick
grub into thinking it is running from that installed system.
EDIT: The directories you listed are needed because in order for GRUB to probe installed systems, it will need access to the devices (
/dev) and to information about system memory, devices mounted, hardware configuration (
/proc). I don't see why you need
/run but that is a relatively new addition to Linux and I am not very familiar with it.