According to this tutorial, by exploiting quadrature sampling with IQ signals, we then have the benefit that each ADC operates at half the sampling rate of standard real-signal sampling (This conclusion in the link is right below the Figure. 18). Why is that? It seems the bandwidth of the signal is the same.
Let’s say you wanted to sample an analog bandpass signal whose bandwidth is 1 Mhz. Using a single A/D converter, to avoid aliasing errors you would have to sample that analog bandpass signal at a sample rate of just a bit greater than 2 MHz (2 megasamples/second). In quadrature sampling using two A/D converters (the top portion of Figure 14), to avoid aliasing errors each converter would operate at a sample rate of just a bit greater than 1 MHz (1 megasamples/second).