You’re not really getting to test out those bodies and lenses, of course, but it’s an evolution of the simulators already available on the web. They lets users see the different field of view at different focal lengths on different sized sensors. The simulators also allow you to see some real-time feedback on how the settings you choose will affect the shot.
Canon have now announced a new VR camera simulator for Oculus Rift. Such tools have been around on the web for a long time. VR does offer the advantage of slightly more realism, though. You can actually turn and look around the scene to find a composition you like. You can even teleport around the scene and see it from a different angle completely. That’s much better than the sets of pre-shot static images you see on web simulators.