The recommended weight for the case is ~10kg. I wouldn't recommend carrying over ~13kg (rack + top case) although I never heard of this case coming off.
Generally, I run out of space before I even reach the 10kg and I also travel with a pillion, so the rack made a lot of sense to me.
My main uses for the rack are:
Carrying fragile stuff tied to it that otherwise would be damaged / scratched if loose inside the case.
Carrying snacks and drinks to consume during a trip (for example a small thermal bag).
Carrying stuff I might use frequently during a trip like tools to adjust the chain, degreaser, chain lube, raincoat