basically there are two reasons for using the capacitors with the crystal circuit
1. The oscillator consists of the inverter inside the PIC, the crystal, and external capacitance . The total phase shift around the loop (from one inverter terminal, through the inverter, across the crystal/capacitor network, back into the inverter) has to be either 0 or 360 (the same thing) degrees for oscillation. The capacitance adjusts the phase shift of the network to allow oscillation.
2. Crystals are designed to "see" a certain type of load, referred to as the Load Capacitance. In order for your crystal to operate at the correct frequency, it must see this value of capacitance at its terminals