As you probably know, the sensation of falling in a dream and waking up suddenly is something of a universal experience- it crosses language and culture as something we all know and feel at one time or another. The science behind that sensation theorizes it was an evolutionary defense mechanism from when humans/apes still lived in trees, because falling meant instant vulnerability to predators. That would put a lot of emphasis on this "rush" as a force of good, or rather necessity, in our lives.
Given the obvious dream-like nature of Kat's world, how pedestrians phase in and out and a world is built without any nearby natural resources, also the nature of the creators shaping a world referred to as someone's "dream", I feel like the gravity powers at work are some kind of fail-safe for a dream out of control. Kat inheriting these powers as a "hero" would then be symbolic of her role as this world's protector, perhaps even the avatar or guardian of the dreamer herself (especially because of her previous position of royalty). Getting cast out of the highest point would make sense if her duty put her at odds with her own people (ending the dream = ending those she is "protecting")
I'm probably reaching here, but I saw the "world pillar" or whatever it was as a metaphor for the spinal chord holding the "world" together, ascending all experience into the brain, which would be the highest, rawest place in a dream. It also sets the stage for the different dream worlds a person can experience in one night, as well as the completely distorted sense of time we have while dreaming. The pillar also obviously calls on several different mythologies, and the gods (the heart of creation) are always based out of the top.
Are the Nevi nightmares taking over, or a natural part of a dream's cycle trying to keep the dreamer from sleeping in too late? Is the great darkness at the bottom an end to the dream, or something more sinister Kat/the Nevi are meant to protect? We don't really have any answers, but I find all the potential fascinating