Jenkins still does stuff that you can't do with GH Actions. Actions ate Travis / TeamCity / CircleCI, all the "more polished Jenkins for the 80% use case" products.
Based on the last time I looked: good handling of dependencies between builds (e.g. the ability to do an "edge build" where for any change in a given project, you check whether that will break your other projects when they upgrade to depend on that), advanced scheduling, plugins that integrate all sorts of random tools into your build views.
It and (GitLab CI) ate Jenkins