Final answer:
Marking a user action as a "key user action" is useful to prioritize it in performance monitoring, signaling the tool to closely track these critical actions, which helps in identifying and optimizing performance issues.
Step-by-step explanation:
One reason it is useful to mark a user action as a "key user action" is b) To prioritize it in performance monitoring. Key user actions are those that are critical to the success of your application or website. By specifying an action as a key user action, you signal the performance monitoring tool to track these actions more closely, which can then highlight performance issues or successes on these critical paths. This ensures that you're aware of how well the most important aspects of your application are functioning from the user's perspective and allows you to optimize them to improve overall user experience.