Quartz Crystal with or without metal lid for EMI suppression?
A crystal like this works at quite low voltage/current/power levels, and they usually have sine wave signal at the terminals, so the protection exists more to protect the sensitive crystal circuitry from external interference.
So in a harsh environment with lots of EMI, it might make a difference, but in that case a crystal with metal shield is not enough by itself to solve the issue, as the PCB design is just as important.