GRUB, no doubt is the most widely used Boot Loader on Linux. So, a system's security is incomplete until its Boot Loader is secure. The common way to secure GRUB is, “password”. But, a “cunning” attacker can by-pass this security also and may gain access to your system, if password is NOT applied correctly to the “grub.conf” entries.
The main reasons to Password Protect the GRUB Boot Loader are: