Quartz crystal oscillators are the high and mighty option for low phase noise and added frequency stability in circuit design. Yes, simple oscillators like those made with resistor-capacitor (RC) or inductor-capacitor (IC) resonators are fine for some circuits. But if you’re dealing with higher performance applications in aerospace, military, and space industries… you’re going to want a higher performance crystal oscillator that can maintain low phase noise & strong stability. Otherwise, you’ll risk deviating from the very specific (and many times critical) center frequency selected for your design.