That’s what genuinely baffles me. If I were a manager of a gardening company and I wanted to do targeted advertising I would just buy space in gardening themed publications. The idea of showing people my ads while they’re on other websites and not in a gardening mood anyway makes literally no sense. All this tracking and profiling ads zero value for the ad buyers, so why are they paying for it?
You're extrapolating based on your own actions rather than measuring the result. The results say that if you show gardening gloves to enough people at random, you'll eventually get some sales. Everything else is just narrowing down "random" a little bit.
Brand awareness/clout. A reminder that you need or want that thing.
And even if I like something seeing it repeatedly online just annoys me more than it inspires me to buy their stuff. I think they'd get more bang for their buck by having social media personalities that align with their target market push their product (think: Nike).
If people are likely to be consumers of those things then they'll probably digest media related to those things or other media in the same demographic. This is classic marketing and it doesn't require the level of privacy abuse that Facebook has been trying to justify.