I wanted to be able to boot all operating systems without having to go through multiple menus. I know I can install NT on top of win9x on top of DOS. I would have to go through the NT menu then the win9x menu in order to boot DOS. I wanted to be able to boot all these operating systems at once.
This turned out to be quite a challenge. The problem with Microsoft operating system is that they all want to boot from the primary partition. This is where GRUB comes in. It can hide primary partitions. You can use up to 3 partitions to install Microsoft operating systems. GRUB will hide the other 2 partitions so that the operating systems will not see it. This means you will need another partition to share data between DOS, Win9x and Windows 2000. The 4th partition is used for the extended partition.
I also wanted a menu system and GRUB provides a nice one.
Another nice feature of GRUB is that it supports reiserfs so I don't need to keep my /boot file in a separate ext2 partition.