Well, for example, a memory chip is a vector of data, but you cannot afford to lay out direct tracks from every memory byte to every location it must go to, so you send the data over a bus.
Another example: you may have a bunch of data to transfer to or from an FPGA, but FPGAs have limited numbers of pins, so you make some of the pins in to a bus and send it in sequence.
Simulink usually abstractly sends each signal is parallel to where it needs to go, all flowing at the same rate for each time step. But you cannot always afford that many signal traces, so sometimes you have to limit the number of parallel paths and send the data in sequence over the restricted path, with that multiplexing happening within each time step.
