3 of my shocks were not leaking when I sent them in for rebuild. The other was damaged to the point that the arms moved without resistance. Since the car has about 175,000 miles on it, I figured I would have all four rebuild, even though only one 'needed it'. What a pleasant surprise when I installed the rebuilt units. I expected the elimination of the tire bounce from the broken shock, but the car really rode and handled like new. It was a remarkable change.
I don't know exactly what they did when they rebuilt them, but it made a hugh change. In my opinion, if you have high milage shocks and even though they are not leaking, they will still benefit from a rebuild. At least that was my experience.