Why do we need to amplify the signal of an electret?
Arduino (or any chip that uses ADC for that matter) has a resolution. For example, arduino analog read divides 5V to 1024 levels. A.k.a 0V -> 0, 5V->1024. That gives you 4.8mV per level.

Now assume that you want to read the voltage fluctuation of a very low voltage power supply that fluctuates from 0 mV to ~8mV. What does the arduino read? You'll see either 0 or 1. So basically, you just can't successfully read that low voltage with your current setup.

Now, if you use an analog OPAMP and amplify that signal to 5V, (0mV->0V, 8mV->5V) then you can use all 1024 levels available to you, provided by arduino.

That's why you need to amplify the signal.