What is the resonant frequency of a cavity?
Signal paths are designed to be low Q resonators. This prevents ringing. But cavities, composed of a power and ground plane for example, can have very high Qs. This means even a little bit of coupling from signal paths can drive resonances and give rise to long range noise voltages within the cavity. Knowing the cavity resonant frequencies you might expect to see can warn of potential problems while you can still fix them.