Comment by om2

Battery friendly because notifications can be coalesced by the OS and processed without having to fire up a full browser engine process and JS VM just to unpack the notification and post a visible notification.

Even in the case where the app needs local processing to show the best notification, having this as fallback removes the risk that the app misses the deadline to display a visible notification and therefore loses its push subscription (which is a behavior Chrome and Firefox have too).

We're also not removing classic Web Push, so web apps can deny themselves the benefits of Declarative Web Push if they don't like it.