8 bits Type-D Flip-Flop, single
This component is used to memorize the state of 8 bits (a full byte) in a single operation. Combined togheter with a "CD4028B 3-4bits BCD to 8-10 IO decoder", the 8 bits Type-D Flip-Flop would allow to select a particular memory bank, ROM, device to talk to over the buses.
Over the bank swithcing oeration, this component is also capable to store a full byte. As an example, this can be a configuration byte where each of the bits can alter the way of working of the machine -OR- store a byte value for a future operation on the arithmetic and logic unit (ALU).
The 8 bits Type-D Flip-Flop is an essential component to creates smart retro-computer solution.
Having 8 bits hardware storage can also allow more evolves hardware operation as we can work on the smallest data size (a byte).