Basically, file systems are stored on the disks.
Almost all disks can be divided up into multiple partitions with independent file systems on each partition.
Here, in the partition of the disk, Sector 0 is called as Master Boot Record (MBR), is used to boot the computer system.
The Master Boot Record's end contains the partition table.
That partition table gives the starting and ending addresses of each partition of the disk.
From those partitions in the table, one is marked as active. So that, whenever the computer system is booted up, the BIOS reads in and executes the Master Boot Record.
The very first thing that the master boot record program does is, locate the active partition, read in its first block, that is called as the boot block and execute it.
Now the program present inside the boot block loads the OS that contained in that partition.
For the purpose of uniformity, each and every partition starts with a boot block, even if it doesn't contain a bootable OS.