Search keywords tell you what the user is looking for. It could be extracted from referral urls coming from search engines, collected from the search box present on your site. These keywords could be stored in the system as part of the contacts profile information.
It could be used
1)For Search Engine Optimization of your web pages.
2)Dynamically change the page content based on these keywords
3)In the email subject to increase open rates
4)In customizing content to be sent to the user
5)Segmenting contacts based on keywords