Sometimes it is necessary to integrate Eclipse with JDK. For instance when some Eclipse plugin requires it, or if you want to easily see Java sources or if you want to have access to Javadoc even if you are offline.
For sources and Javadoc is important, that when you download and install JDK, you will also have Java sources on your hard drive.
Now we must just add JDK to Eclipse and make sure that a project has in Java Build Path selected this JDK.