What’s a benefit of having multiple ads in that ad group?

Ads are only eligible to show ad extensions if there are more than one ad in that ad group
Ads are only eligible to show at the top of the page if there are more than one ad in that ad group
AdWords will automatically rotate your ads and show the best performing ones more often
AdWords will automatically match each ad to the keywords it’s most relevant to

Explanation: 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

See all AdWords Fundamentals Questions and Answers here.