Google AdWords offers four ad rotation options:
Optimize for clicks: gives preference to ads that are expected to attract more clicks than other ads in the ad group, based on your past clickthrough rates (CTRs).
Optimize for conversions: gives preference to ads that are expected to attract more clicks than other ads in the ad group, based on your past clickthrough rates (CTRs). : gives preference to ads that are expected to provide more conversions, like purchases and sign-ups.
Rotate evenly: delivers your ads more evenly into the ad auction. Ads rotate evenly for a 90-day period. After the 90-day period, the campaign will automatically begin to optimize for clicks or conversions.
Rotate indefinitely: Delivers your ads more evenly into the ad auction, but does so for an indefinite amount of time and does not optimize.
Ad rotation is the way Google delivers your ads on both the Search Network and the Display Network. If you have multiple ads within an adgroup, your ads will rotate because no more than one ad from your account can show at a time.
The recommended approach is either optimize for clicks or optimize for conversions. But rotating evenly is a good way to test different ad copy against each other. So if you are testing different messages, value points and calls to action within the same ad group, rotating evenly is a good way to set up a rough A/B test to see which ad outperforms the other(s).
Google states: “An optimized ad rotation automatically rotates in the creative that is projected to perform the best in each auction. Ads optimized to drive clicks or conversions can improve both your competitiveness and your bottom line. Every impression that an underperforming ad receives is a missed opportunity, and those opportunities cost you money.
Ultimately, when you’re deciding the outcome of ad tests, you can use a combination of performance stats like CTR and conversion rate along with percent served. If the system is showing an ad more often than another, it can mean that it’s better at winning impressions in the auction itself, which is often a strong reason to prefer that creative over others.”