So then consider that when you go to other websites, with FB code embedded within them, some of them e-commerce sites, they will be able to retarget those ads, which you already showed at least slight interest in. This is likely a huge difference.

You already showed interest in that gaming mouse, but for whatever reason you decided you would buy it later or not now, maybe forgetting all about it eventually, but then you see that ad to remind you. That's a huge difference.