By tracking these actions, known as "conversions," you'll know which ads, keywords and campaigns bring you business. This helps you invest more wisely in the best ones and ultimately, boost your return on investment (ROI).
https://support.google.com/adwords/answer/1722022?hl=en
The Offline Solution
With the AdWords Conversion Import feature, the seller can add the sale into the system long after the original click has occurred. This can be weeks later but the conversion can still be followed.
This also allows companies to track repeat customers by recording a conversion online and then tracking additional sales which may occur offline. In this way, businesses can increase their reach by better understanding how their target market is reacting to their advertising.
http://unbounce.com/ppc/new-google-adwords-offline-conversion-tracking/