...at least that's the theory. Apparently there is some reason to believe that the cat purring helps keeps cats bones healthy even though they are often giant couch potatoes (and bed potatoes, and floor potatoes, and windowsill potatoes). The subtle body vibration may help increase bone density. And since astronauts experience bone density loss due to the difference (or lack) of gravity in space, they have been experimenting with ways (in addition to post-space rehab) to try to adjust for that. And, as this article suggests, maybe a cat purr is called for here.
h/t to the tweeple who pointed me to this link.