See Difference between installing GRUB on MBR sector or first sector on boot partition? and GRUB in MBR or Partition?
Basically, if you wanted to use some other bootloader to chain load GRUB, you would install GRUB onto a primary partition instead of the MBR. This might be the case if you are using multiple operating systems, and wanted to use a different bootloader in the MBR (there can only be one there). Also, hardware manufacturers sometimes include code in the MBR, and by overwriting with GRUB you might wipe this out.
However, there must be some bootloader in the MBR, so if GRUB is the only bootloader on your system, it needs to go there. Generally, if possible, you should install GRUB to the MBR.