Natural search can adjust your Adwords focus: Google Analytics will show the keywords and the full search phrases bringing visitors to your site – often opening your eyes to the questions people are truly asking and how they are thinking.
Google Adwords can guide natural SEO: If you optimize your site for top performing paid phrases you can reduce your dependence on paid search by snagging more of that traffic naturally. Also, the great tools that help you optimize Adwords by showing how many actual searches are performed for specific phrases can help you focus natural search work where traffic definitely exists.
The bottom line is that both paid and natural search cost money and resources – and your mission is to generate the best overall marketing ROI. However, their cost accrues differently: Adwords is like paying rent, while natural search is an investment in ownership. You may always need both, but by fueling each other, you will optimize total search performance as cost-effectively as possible.