navigation.activation stores state about when the current Document was activated (e.g., when it was initialized, or restored from the back/forward cache).
In some cases, like in view transition, developers want to customize the page/style/what not based on the last/current navigation: which URL did it come from, was it a back navigation, etc. For example, show a welcome message but not if it's a back navigation, or run a different animation if coming from the home page.
Explainers: https://github.com/WICG/view-transitions/blob/main/navigation-activation-explainer.md