> it is almost always is something I don't want, already have, or just bought.

The explanation I have heard for the "just bought" scenario (I heard it on the internet, so grain of salt and so on) is that they're looking to catch people who purchased the item but have returned it, and are now in the market for a different version of the same item (e.g., you returned the chainsaw because it was too small, and now want a bigger one). Which makes sense, but whether there are enough people in that scenario to be worth the cost...

That seems unlikely, when you consider the advertising market and how it works. Fundamentally, the providers (i.e. the chainsaw store) don't tell the advertising platforms when a person buys a chainsaw (and often they can't tell themselves).

So, what ends up happening is the situation that many people complain about is that the provider shared a list of people who looked at chainsaws, and then the advertising platform shows them ads until they run out of money/find more profitable ads to sell.

From the perspective of the ad platform, it doesn't matter that you already bought it. From the perspective of the provider, this is acceptable waste.

It might be that personalised ads are only marginally better (if at all), but the perception of the advertisers and publishers is that it’s very important, thus creating a (fakeish) moat for Google, as it’s more difficult to execute. Or maybe that’s really not that important, as the biggest moat remains Google being number one web destination.

For us advertising a product with targeted ads definitely performs better than non-targeted ads. Also remember that most stuff being advertised is consumable so you will most likely at some point buy it again.