How does enlarged lymph node removal for microscopic exam aid in diagnosing certain disease?

It's very helpful: Looking at the lymph node under a microscope can make the diagnosis if it's uncertain. If there is cancer present, then the pathologist can actually see the cancer cells and ascertain where the cancer is coming from and how aggressive it is. There are other causes of enlarged lns that can be identified as well - e.g. Fungi, or granulomatous diseases - these can be seen many times by microscopy.