Although Chu Yu didn't plan on harming You Lan, she intentionally prolonged the time of her fear, which was a form of psychological oppression and a small warning to the young girl.
Don't be too presumptuous, don't make decisions on your own, were the implicit messages from Chu Yu.
Every person is an independent entity. Chu Yu never believed that by merely intimidating or shouting about equality, she could easily win people's hearts. Interests, emotions, rationality, desires—people are composed of multifaceted factors. Sometimes, so-called loyalty is just a byproduct of interests. Everyone has their own motives, including Chu Yu herself.
If she herself was not above such things, on what grounds could she expect others to become puppets without self-awareness?
Thus, Chu Yu could fully understand You Lan's little schemes. Although it seemed like Rong Zhi had fallen out of favor, the accumulated authority over time still made You Lan subconsciously obey him.