Resistor symbol in a schematic with zero value
CMOS crystal oscillators may require such a resistor, because the amplifier
that drives the crystal (a biased CMOS inverter) has low output impedance
when it is either HIGH or LOW.   That low output impedance is a short
circuit across (effectively) the load capacitor connected to the
crystal at that output terminal.  That has the effect of damping the
oscillation.

Mainly, modern processes allow weakly conducting CMOS inverters for this
function, and most application sheets do not call for a resistor at the
driven node of the CMOS gate.  If an unusual impedance of crystal or
a different frequency range is used, either the 33 pF capacitors or the
zero-ohm resistor may prove unsuitable, so it may be prudent to allow
for changing those values.

It would also allow disconnecting the onboard crystal for diagnosis (temporary)
or customization (permanent) purposes.