You could imagine a Venn Diagram of the two extension APIs. One circle is Chrome's, the other WebExtensions. The overlapping are are basic extension functionality (''give some examples'').
Outside of that overlap area would be the areas of differentiation. For example, we don't have much interest in providing easy access to Google's services though WebExtensions. We do however want to support certain use cases that don't seem to be a priority for Google, . We have identified a few such areas that we want to support such as ad-blocking, tab management , UI modification and time/location shifting content.
=== Why WebExtensions and not Jetpack? ===