Here's a good way of thinking of it: I/Q is twice the amount of information as just I. A complex sample rate of 1kS/s is equivalent to a real sample rate of 2kS/s.
An example for amateur radio would be that if you modulate a real carrier with real information, you get a signal that is mirrored around the carrier (e.g., AM, DSBSC). There is redundant information. You can prove this by demodulating the signal in SSB (with perfect tuning). The signal is perfectly readable. If you modulate a complex carrier with complex information, there is no mirroring. SSB (relative to the carrier) is a complex signal that happens to have one unused side. It uses half as much bandwidth to carry the same information.
Hardware is also a driver. It is cheaper to sample two signals, offset by 90 degrees, at half the sample rate, for instance.