What is one reason it is useful to mark a user action as a "key user action"?

Marking a user action as a "key user action" primarily serves to extend the retention time of the action's full historical data. This feature is particularly beneficial for businesses that want to analyze the performance and user experience related to crucial interactions over an extended period. By categorizing an action as key, Dynatrace allows that action's metrics and information to be preserved longer than standard data, enabling users to conduct thorough analyses and track changes in performance or user behavior over time. This can be crucial for identifying trends, understanding user engagement, and making informed decisions to improve the application.

The other options, while they may contribute to the overall effectiveness of monitoring and assessing user actions, do not specifically highlight the primary benefit associated with marking an action as key. Improving usability, enhancing speed of retrieval, or facilitating reporting are valuable, but they are not the central focus of this particular designation within Dynatrace's functionality.