Why does Katarina get a cooldown refund(reset) for every assist or a kill?

^Title

I don't think Katarina is broken and this is not a QQ thread.. one hard cc her ult stops and she gets blown up in teamfights. I know this for a fact because I play her myself, but why is she, from a design perspective, get a cooldown refund for every kill or assist? All assassins have to wait for at least couple seconds before they can get their full combo back up from cd. But Katarina can just keep going without waiting for any cd once she gets going. I mean its not like she's any less mobile or does less damage than other assassins(both ad and ap) in the game.