how is output impedance useful
It's not always useful, in fact it's a hindrance sometimes but you have to live with what the circuit is and does.
The circuit inside the microphone is just a JFET amplifier and that converts the tiny microphone voltage to current and this current flows through RL and up to the supply voltage Vs. The article linked to for the microphone is for a 2 pin microphone that would require an external "2k2" going to a battery or supply voltage. Here is wiki showing a typical circuit. The "2k2" is in series with the drain of the JFET and this is what the output voltage develops across.
So the tiny input voltage is turned to current and the resistor RL turns it back to voltage BUT with a decent chunk of amplification. The simplicity of this circuit comes with the drawback that there is an output impedance that should not be shunted too much by the input impedance of the amplifier it connects to. If the input impedance of the amp were 2k2, the output voltage from the microphone would be halved compared to an amp with +100kohm input impedance.
There are three terminals on the microphone so why didn't they use something like an op-amp internally with the third terminal being the output. The op-amp would have very little output impedance and you could probably run it into an amp with input impedance as low as 100 ohms without too much trouble.
But, on the other hand who cares; nobody intentionally designs audio amps with low input impedances but if they need to be that way then that's just as easy as making them +100k. Using a JFET in the mic is dead easy and it's suprisingly good on hifi being class A with no chance of cross-over distortion or feedback (that supposedly causes intermodulation distortion we can apparently hear LOL).