Both Google AdWords and Analytics offer insights related to the performance of your AdWords campaigns, and those insights get better when you use Google Analytics + AdWords together. Improve your bidding, messaging and performance through these analyses.
The Importance of Linking AdWords and Google Analytics Accounts – Google Best Practices
Optimizing AdWords with Analytics Goals and Audiences – Google Best Practices
Using Google Analytics Metrics in AdWords Reporting – Google Best Practices
Analyze AdWords Performance in Google Analytics – Google Best Practices
Optimize & Measure: Quick Tools to Supercharge AdWords Campaigns