The keyword menu comes up on the edit hub/stats for hub menu.
When you first write a hub the keyword menu is blank, and appears useless. Go back after a couple of months however and you will find the keyword menu full of interesting phrases and words which Google has linked with your hub.
This then raised the question, What do you do with this information?
You can use the information in several ways. You can edit and re-write parts of your hub to include one or two of the keywords and key phrases that Google has provided you with.
You can write a hub on a similar topic and use the keywords already supplied with Googles report.
Personally I like to go back and tweek my hubs after about four months. By this time they are maturing and the longevity is making them appeal to Search Engines they have often been read by several hundred visitors and you can also incorporate some of the comments into the hub to add extra interest or answer questions in the hub itself. This saves the new reader having to look at all your comments for a interesting tit-bit of information.
Sometimes I have found that I have the makings of another hub, the list of keywords is useful to tag the new hub and also build a good framework to expand upon it.