I believe the buffer is the memory chip on the hard drive where information is stored before writing to the drive or reading from it. The bigger the buffer the more data can be held when being written [Same as burning a CD] also data can be accessed or read from this memory chip.
Access to the memory chip for reading/writing is far faster than from the drive itself.
I think I've got that right? Someone will correct if I have misunderstood the function and reason for the capacity of the memory chip on the drive.