Data buffering is quite helpful for purpose of smoothing out gaps in speed of processor and I/O devices. Data buffers are registers that hold I/O information temporarily. I/O is performed in short bursts in that data are stored in buffer area whereas the device can take its own time to accept them. In I/O device to processor transfer, data are first transferred to buffer and then passed on to processor from these buffer registers. So I/O operation doesn't tie up bus for slower I/O devices.