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3. To improve your ads.
By testing different versions of your ad, you will find the most effective one. To obtain a true test of a particular ad, you will need to know how may people were shown your ad, how many people acted on your ad, and the result of that action. (For example, your ad was shown to 1000 people of which 100 clicked on your ad link and 2 bought your product.)
4. To find out what works and what doesn't.