Answer:
if there is a particular ad you are referring to, PLEASE ATTACH IT so I can better answer your question.
In general,
the purpose of an ad can be:
to persuade the audience to participate in their product or service like clothing ads or an ad for a new store for example.
to persuade them to practice something or stay away from something such as a healthy eating ad or a PSA ad on staying away from drugs for example.
It really depends on the type of ad and the purpose of the ad.