Normally an inverting amplifier will not have the 100k and 1k resistors. 100k= short, 1k=open. When the signal generator outputs 1 volt the amplifier will output –1 volt to return the input back to ground. Because the amplifier does not have infinite gain the output is slightly less than 1 volt and there is a very small signal the (-) input. If the amplifier has an open loop gain of 10,000 then the signal at (-) input will be 1/10,000 volts. That is hard to measure.
By making a 1/101 in front of the amplifier (– input) we can see the effect of open loop gain multiplied 101 times. (The signal at (-) input is still 1/10,000. The signal at the junction of the two 10k resistors is 101 times bigger. Approx 1/100.