Why are the capacitors necessary for a quartz crystal?
Ok, I got the question. The answer is:

Crystals must be connected in a feedback loop of some amplifier, in some way or another. As such, the crystal will be subjected to parasitic circuit and board capacitances, aka "loaded". One can't avoid this "load".

At the same time the frequency of this "feedback" filter [electro-mechanical resonance] depends on the load capacitance, to a certain degree (called "crystal pullability", about 8ppm per 1pF of load). Therefore, to get a well-defined frequency of oscillations, all crystals are tuned at certain specific load, 12pF, 20pf, etc. during manufacturing stage, which becomes a part of crystal specification. The task of a designer is to meet these specifications if they want a good specified frequency.

There are two caps because the load is effectively split between output capacitance and input capacitance in the typical Pierce Oscillator schema. So the caps are essentially connected in-series. Thus, a crystal specified for 20 pF load should use two 40 pF caps.

Now, a good circuit designer understands that the IC pins have certain inherent capacitance (2-4 pF), and PCB traces and pads also have some capacitance (3-5pF, depending on particular layout). So these parasitic capacitances be better accounted in the circuit, so the actual caps are usually smaller than the 40pF as per example above, and could be 22-27pF after all corrections. In some cases no caps are required if the pin/trace/pad capacitance already meets the crystal specs.