Its important to remember that the CPU runs at a much faster speed than a storage device, like a hard disk, can actually store the data. Hard disks have physically moving parts which slow it down. CPU's and Memory (RAM) have no moving parts. This is why the use of a buffer is important so that the CPU does not have to wait for the storage device to finish before it starts working on another task. While the saving of the file is taking place, the CPU will be carrying out many different tasks until it gets an interrupt requesting for the buffer to be refilled.