I think the key line is this: These experiments help[ed] to improve understanding of how the body’s sensory systems work when the usual gravitational cues from

I think the key line is this: These experiments help[ed] to improve understanding of how the body’s sensory systems work when the usual gravitational cues from the inner ear aren't available In a zero gravity environment, you won't have reference from the inner ear. These volunteers also did not have that reference due to their spinal menigitis, so it allowed a way to observe how the body's sensory systems behave in such a scenario.