Paul B. is right. The devices he's talking about are simple shift registers.
Some of them are buffered and will require another bit to transfer the data
from the buffer to the output of the shift register, this is handy when the
data is destined for divices that can't have data changing at the input.
Without the buffer for example the least significant bit could change each
time a more significant bit passes by. Might be ok for slow devices like
solenoids.

Good Luck

Steve