To save his good friend Suzaku Kururugi, Lelouch had paid a considerable price. Although Lelouch had known from the beginning that his sister Nanalie would inevitably have to be sent to Xiao Ran, he had taken the opportunity of leaving his sister in Xiao Ran's care to make a request of his own. At this time, it made it seem more like Lelouch had sent his sister over solely to save his good friend.
Although Lelouch knew that feeling was merely an illusion, it still made him, someone who loved his sister dearly, feel somewhat uncomfortable. Of course, Xiao Ran would not regard Lelouch's request for help in rescuing Suzaku Kururugi as a simple transaction. This was part of the cooperation and deal between them, nothing more than an insignificant detail.
Still, when Lelouch heard Xiao Ran agree to help without hesitation, he at least felt a sense of relief toward this inexplicable partner of his.
As for why Xiao Ran had agreed to help Lelouch rescue Suzaku Kururugi, it had been a casual decision. He and Lelouch were still cooperating, and since they were partners, it was only natural for Xiao Ran to provide support when one side encountered trouble. Besides, even without Xiao Ran, Lelouch would have found a way to rescue Suzaku Kururugi anyway. Refusing would only create resentment and ill will out of nowhere, so it was better to agree decisively. At least Lelouch would be more cooperative in their future dealings.
Xiao Ran also happened to have arrangements ahead that would require Suzaku Kururugi, so this was a good opportunity to get in touch with him and see whether he could begin exerting some influence and control over him. Since there was no other way for the time being to smoothly join the Empire, replacing