Final answer:
The Count advised Valentine to pretend to be asleep most likely for strategic reasons such as to protect her or to gain information without being detected.
Step-by-step explanation:
You probably meant to say You can never wake a person who pretends to be asleep. Meaning, you can never get the person to hear your requests because he is not merely ignoring you out of distraction towards some other business, but does that on purpose, only pretending to be distracted by something.
The Count likely advised Valentine to pretend to be asleep as a strategy to gather information or for her protection. In many literary works, a character might feign sleep to overhear conversations, to avoid unwanted interactions, or as a way to observe a situation without influencing it directly. Although the context of this directive isn't provided, this kind of advice would typically serve to safeguard Valentine or position her advantageously within the plot.