I think you summarized it well.This button is one of the features of Google that makes it different from other search engines. If you think the keyword you have entered is accurate. And it will take you to the site you are looking for then you can simply click on I'm Feeling Lucky button. What it will do? It will just take you to the site which is ranked on that keyword. Mostly we are not taken to our desired site. However for a researcher who knows the right keyword for a site. He can easily use this feature for his benefit.