Chainloading an operating system allows grub to boot an opearating system's boot loader. This is commonly used to boot Windows for instance. I personnaly use it to be able to have my "production" system's grub on the MBR, and address other distros'grub install on their root partition. The advantage is that kernel updates are real easy to handle. Each testing distro modifying their own grub won't interfere with my main OS bootloader.
This tutorial will explain how to configure grub's menu.lst from the main OS to boot other OSes bootloader through an example.
By being able to boot another grub from a primary grub, it make it easier to keep every OS kernel updated, as anytime a distro is updating its kernel, its own grub menu will be modified to represent the changes.