Facebook has multiple layers of machine learning models and rankings. It tries to find best potential customers for your product based on their social signals.
A/B testing allows you to change variables of your campaign and particular ad to figure out which appeals the best for public. These variables might be image, click to action text, target audience and many more. So this allows to save your budget and show the best performing advertisement to large scale audience.
If you have several ideas and need to make decision, A/B testing is best solution to compare them and reveal the winner for further promotion.
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