Google needs to keep its loyal advertisers and coach them the best techniques and tweaking strategies to enhance their site and its ROI and CPA. Google created Google Website Optimizer ( optimiser ) what we all as search engine marketer knows as usability factors that increase your selling process in less time. You need the visitors to buy or get the highest trust towards the ads that he clicked on and take less time to buy the product.

Why does Google worries about all of this, it is just because they need to keep their advertisers happy enough and use later on their Google Checkout ( the more Google can teach you, the more money google can get in return), what I call Knowledge process outsourcing or sharing.
Two things to keep in mind:

1. less time for your visitor to make a decision ( trust, usability factors, easy personlized features)
2. higher CPA ( CTR, time, check out )