First-click metrics are vital in evaluating tree testing results, as they provide insights into user navigation behavior. The first click is the initial category users select when starting a task. In tree testing, this is always a top-level category because subcategories aren't visible until a parent category is selected.

When a task has a low success rate or directness, examining first clicks can reveal where users initially expect to find information. For example, if users consistently start in "Support" when looking for "User Guides," it suggests that these categories are too similar and might benefit from clearer labeling or separation.