What’s a benefit of having multiple ads in an ad group?
AdWords will automatically match each ad to the keywords it’s most relevant to
Ads are only eligible to show ad extensions if there are more than one ad in that group
Ads are only eligible to show at the top of the page if there are more than one ad in that ad group
When using optimize for clicks, AdWords will automatically rotate your ads and give the best performing ones a better chance of showing more often
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 ad group, your ads will rotate because no more than one ad from your account can show at a time. Use the ad rotation setting to specify how often you’d like the ads in your ad group to be served relative to one another.
A preference that determines which ad in your ad group should show (when you have multiple ads). Ad rotation settings are helpful because they give you the ability to request how your ads are shown. For example, you can request to have your successful ads show more often or indicate that you’d like your ads to be shown more evenly.
If you have more than one ad in your ad group, Google will rotate which ad shows depending on your ad rotation setting.
Read more here: https://support.google.com/adwords/answer/112876?hl=en