The search monitor allows you to see what other people on the GnutellaNet are searching for. It is certainly instructive to do this once or twice just to see what can be seen. It can be entertaining if you are bored. Don't do it for too long though, it might get you down on people. I am sure all the people searching for "back street boys" and "lesbian granny" are using MLDonkey.
In either leaf or ultrapeer you can observe searches that are routed to your computer. When you are a leaf node, this gives you an idea of how well the GnutellaNet is targeting searches. Many of the search terms should be related to files you share.