Most likely only the designers know that and the info might not be public, so we have to guess.
Inductor and capacitor is used as a LC filter. The filter removes noise and ripple that might come from the power supply or the chip digital supply pins, so that the voltage that powers the sensitive analog power supply for the clock has less noise and ripple so it can generate cleaner clock.
The capacitor and inductor values are designed to result into suitable low pass cut-off frequency so DC is passed and high frequency AC that might start to affect the cleanliness of the clock is filtered away. As the inductor increases the power supply impedance, the capacitor must be large enough to bring the impedance down to a suitable level so the ripply currents taken by the filtered node do not result into too high ripply voltage at the capacitor.
And since LC filters exhibit resonance and ringing if not properly damped, the resistor might be added to introduce just enough losses to damp resonances and ringing enough to not be a problem.