What is the purpose of the advertisement?

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.

it's no easy i swear it's hard