It is required for two reasons as follows:

1) An internal crystal bias setting sets the crystal drive level of the 24MHz crystal. This bias setting is calibrated to be the lowest possible level for stable oscillation. (The lower the setting the lower the current consumption.) The bias calibration occurs at power up. As the crystal drive level increases, the oscillator signal level increases. A high signal level will pull the 24MHz oscillation frequency towards the +/- 40ppm limit. Adding a 3.3k Ohm shunt resistor prevents frequency pulling over the allowable the crystal drive levels.

2) The ESR tolerance causes a wide spread of ESR Values across multiple reels of parts. The shunt resistor improves the ESR tolerance of the crystal and prevents a situation where the ESR gets too low. If the ESR is too low, then larger signal swings for the same bias current are present. Larger signal swings increases the risk of clock related noise interferers within the EM250.