CompletableFuture.runAsync(...) runs the Runnable in the forkJoin-Pool which is managed, while new Thread() creates a new thread which you have to manage.
What does “is managed” mean, it’s pre-allocated and the threads are shared in the JVM. When the runnable is completed, the thread can be reused for other runnables. This makes better usage of resources, especially as thread instantiation is an expensive operation – not only the object, but also some extra non-heap memory – the thread stack – has to be allocated.