Original: The disk buffer is controlled by the microcontroller in the hard disk drive, and the page cache is controlled by the computer to which that disk is attached.
Simplified: This interface almost always works at a different speed than that of the hard disk platter. In such a case, the disk buffer is used to store the data that comes from the disk, before it sent to the computer, or to store the data that comes from the computer before it can be written to the disk.