> An advertisement is always designed to try to convince you to buy a different product than you would rationally choose yourself.

Perhaps you could consider toning down the absolutism. This is true in many or most cases, but certainly not all cases. Let's take, for example, video games. I can afford to purchase any game that interests me, and do. However, I often go several months between new game purchases, because I am not aware of any games that interest me that I do not already own. An advertisement for a game does not need to convince me to purchase it over an alternative product, it simply needs to make me aware of its existence and broadly convey what the game is about so that I will know whether it matches my specific game interests closely enough to investigate further.

Particularly in the modern world of hyper-specialised interests, it's quite easy to get into a niche of a hobby where you have found and already purchased all of the things you are aware of. As another example, there are hyper-specific novel genres where there are at most a couple of dozen entries in that genre and you are able to read every single entry in it. You are still interested in that genre, and will likely purchase anything else in it, should you become aware of it. Enter the benevolent advertisement, which makes you aware of its existence in a mutually beneficial way wherein you get more of the content you are interested in consuming and the creator gets money.