All those light-based visual cues that we take for granted, that are extremely limited in most games because it's been too costly simulate, that's where ray tracing has the most gameplay potential. Stealth and detection are the most obvious aspects, and even very conservative uses of ray tracing could benefit here. Simply being able to have accurate shadows would dramatically increase detection ranges, as well as allow for forms of concealment and subterfuge, that would be difficult to implement any other way.
Realistic lighting, or at least a plausible abstraction of it, is also a prerequisite for AI visual systems that behave plausibly. Even the best NPC AI we currently have in first-person titles cannot behave plausibly because the simulated sensory inputs they have don't resemble our own. Often the same characters will seem to have a preternatural awareness in many situations, while being impossibly blind in others. Personally, I've always been a big fan of stealth games, and games with stealth aspects, but the best examples in this genre have glaring problems with AI, and even multiplayer examples suffer due to the inability to render realistic scenes.
At a basic level, it's not really much different from how VR can improve gameplay...both improve visual fidelity, allowing more, and more accurate, information about a game scene to reach us in an intuitive manner. A second view port allows stereoscopic vision and depth perception. Light that behaves as light should actually behave allows it to be used in the same way it's used out of game.