Resistor is required to limit the current flow through the LED since otherwise so much current will pass through that power dissipation shall destroy the LED in less than a second.
For nonmultiplexed display it is quite simple to determine the value of this resistor since... the LED has 1 diode drop when it is turned on, the rest shall be across the resistor. This enables use of simple Ohm's law.
The situation gets complicated for a multiplexed display, now there is voltage drop across the PNP (for common anode) emitter collector junction and the base of this transistor also needs a resistor.
(1) Is the resistor on the base used to limit base current?
(2) How does one determine how much current shall be flowing through the transistor i.e collector current? I don't think the transistor action equations can be used since the transistor is likely in saturation mode. Using beta value will not help. Besides, the base current value is also unknown.
(3) How does one determine the value of the base resistor and the display resistors in this case? The collector-emitter voltage drop is unknown which is creating the complication.