Quite a few boot managers are available. Of course, Linux uses LILO, FreeBSD installs BootEasy, and you can also use OS-BS, payware like System Commander and other boot managers that ship with commercial operating systems like OS/2 and NT. So why GRUB?
OS-neutral: GRUB is not tied to any particular platform, and you don't need to run any special operating system to install and configure it. With LILO, for example, you need to be able to get into a Linux system for setup and maintenance, and remember to run the LILO binary after changing /etc/lilo.conf. GRUB can be set up and maintained from a number of systems, and any changes to its configuration file are immediately available.
Interactive: GRUB offers an interactive and configurable menu interface, and a command-line mode is always available. The command-line features are invaluable for getting into an OS whose boot configuration got creamed. Even boot settings within menu items can be interactively adjusted as necessary from within a GRUB session.
Powerful: If your BIOS handles LBA mode, GRUB can boot kernels beyond the 1024th cylinder, making kernel installations possible in partitions beyond 8GB. GRUB can do tricks with disk mapping and partition hiding, so you can run most OSes from whatever disk you want. GRUB also has network boot capabilities, enabling BOOTP and DHCP from remote servers.
GRUB is actively maintained and under continuing development. Its acronym expresses the goals of its authors to become the unified bootloader, so that every new OS project need not invent its own boot system. Although GRUB is primarily available for i386 systems, ports are underway and in various stages of completion for other architectures as well. By learning to use GRUB, you can be confident in having a single boot manager to handle any OS you may ever choose to install.