The benefit to having multiple cores is that the Garbage Collector (GC) can use a dedicated core in parallel while the application uses other cores and is uninterrupted. This is instead of a Stop the world type of collection, which effects the running application.
Java 7 GC makes use of multiple cores using the G1(-XX:+UseG1GC) collector or by the use of the following Java options that will use 16 cores:
-XX:+UseParallelGC -XX:ParallelGCThreads=16
Some of these settings may be the default in certain environments.