OOver the past decade, Google has fundamentally changed how it categorizes knowledge-moving from strings to things. In other words, Google used to organize pages based on what keywords they contained, without bothering to understand those keywords' meaning. Now, Google can understand how related terms and ideas connect and guess at the intent of a person doing a particular search. If you're still optimizing for only a single primary keyword, you're behind the times, and you're hurting your rankings. Secondary keywords can help.