to slow the vacuum/boost "signal" down so the computer has enough time to adjust it. Otherwise the line would flow too much air and the computer wouldn't be able to keep up.
thats not why. it acts as a flow limiter, yes, but not for a time delay.
without the orifice you are asking the wastegate solenoid to control pressure in a line directly connected to the intake manifold. which means you are asking it to fight the entire power of the engine to maintain that line at boost pressure, limited only by the line diameter and length. not going to happen. the orifice makes it so the engine cant influence the pressure in the line as strongly.
in electricity its the equivalent of a relatively high impedance "resistor" (or vacsistor lol)..like a pull up resistor of a few k on a signal line being controlled by a processor. if it were only 1 ohm the processor is not going to be able to control the voltage on the line because it will be trying to shunt the power rail to ground..boom. doesnt go boom in the engine situation because the solenoid doesn't dissipate power according to the same equation as a resistor.
so when people dont put the orifice in, and the ECU wants more boost, it opens the wastegate solenoid, venting intake manifold pressure to the atmosphere, but the engine acts as an essentially perfect pressure (voltage) source, and some pressure is still in the wastegate line even though tons of air is flowing out the open solenoid..and so the actuator still opens up the swingvalve, and you dont get as much boost.
actually I looked at the diagram in this thread and it doesnt seem to function the way I described. in that case im not sure why its there.
but if you look at the diagram for an 89 daytona, its different. the boost source is connected to the actuator/solenoid circuit through the orifice. that is what i described. why did they change it?