optional boot methods
The Master Boot Record area is at the very beginning of the disk, so is not inside any partition. If you have only one disk and one operating system, it is best to boot from the MBR. But some people want to run more than one OS - like Windows and Linux - on a single disk. I did this for many years. You can either use Grub to boot into both Linux and Windows, or use a Windows utility to boot both Linux and Windows. There are a few advantages to using a Windows boot utility, and if you do, you must install the Grub boot loader within a Linux partition, instead of in the MBR area.
There are more OSes than Linux and Windows, too, and it is possible to run them on the same computer side by side. It is also possible to have numerous disks in the same computer. Unfortunately, you should not have more than one partition of one disk 'active'. Otherwise it might get terribly confusing. So if you are hypothetically running four operating systems on four different disks in the same computer, you will have to install the boot loader for three of them inside a partition, not on the MBR.
Of course, you can also boot from a jump drive, or a network card , too. The possibilities are very numerous. It's hard to think up more scenarios, but they do exist.