This lets you run an existing Java web applet that you already have, from way back when those were supported, in a modern browser with real sandboxing. Even if you don't have the original source. It's for legacy stuff, not new projects.
Their AOT solution was clearly too complex for some potential clients, that seems like the real reason they rewrote it all, support for higher Java version is mostly a neat benefit (though Java 9 did still support Applet from my understanding, just deprecated).
