In wifi modules what does the shield have to protect?
More than likely the shield is preventing any amplified antenna signals coupling back to the Wi-Fi antenna and turning the circuit into a useless oscillator. This is commonly seen in radios of all frequencies and is especially a problem at above a GHz because any short length of track carrying an amplified antenna signal behaves more like a transmissive antenna than at lower frequencies. Just think about the signal gains that are involved in amplifying less than -80 dBm from the antenna.