That’s the idea: an anteroom containing the accoutrements to accompany the Pharaoh on his voyage into the afterlife. According to Houdain’s theory of how they were constructed, the ”Grand Gallery” was basically a machine spade that served no ceremonial purpose whatsoever and was sealed off from the main ceremonial passageways as a bit of an embarrassment. That, in turn, is why we have been oblivious about these other spaces for so long: quite by coincidence, when breaking into the pyramid, the service tunnels were the ones the profaners quite accidentally blundered into. According to Houdain we’ve been crawling around inside the ductwork of the Waldorf Astoria for centuries.
Or maybe another gallery leading to a third chamber. These are my theories.