I wouldn't say it's the only purpose, there's still advertising aimed at customer accquisition, but there are definitely ads with that customer retention goal in mind as well. Capitalist overproduction constantly requires people to buy stuff they do not actually need, or spend too much on a fancier version of something they do need, or buy stuff they do not even really want on an impulse, so buyer's remorse is an expectable (and actually perfectly reasonable) reaction that corporations have to keep in mind.

Ads aimed at "post-rationalization" (telling yourself it's ok to have bought something useless) are a way to combat this buyer's remorse and retain customers, especially for luxury brands who have very few customers, but generate a lot of revenue off each of them. This is why marketing material for Porsche informs people who are already their customers how good the ceramic breaks on their new Porsche are: It convinces them that the midlife crisis mobile they just wasted 200 grand on isn't a death trap they, as usual Porsche owners who drive their Porsche twice a year, cannot handle at all. It reassures them that they made the correct choice and that they are happy with their purchase so they keep their Porsche in their garage and don't resell it while it's still a recent model.

It would make little sense to tell this to potential customers, as nobody has ever bought a Porsche because it has good breaks. It's a car you buy for purely emotional, irrational reasons, but after the purchase logic may kick in and the bit about the breaks is part of a marketing strategy that guides the customer in the "right" direction once this happens.