The onUpdated lifecycle hook in Vue 3 Composition API is called after the component has been re-rendered due to reactive data changes. As the creator of CoreUI with over 11 years of Vue.js development experience, I use onUpdated for DOM measurements, scroll position adjustments, and third-party library updates that need to happen after Vue updates the DOM. This hook is useful when you need to access the updated DOM or perform actions based on data changes.