a completely honest question, what's wrong with recommending a product without mentioning if we're affiliated or not with that product?

It's advertising misleadingly pretending to be not-advertising. If you have a potential motive apart from "I genuinely believe this is the best recommendation I can give you" for making a recommendation, you probably should disclose that. Especially in a community like HN, where talking about your own stuff is somewhere between accepted and encouraged, I can't think of good reasons not to do it.

Thanks for this insight! Now i see the difference and where bias could play a role when recommending a product in case we're affiliated with. In fact, I see this widely used only in the HN community.