Right?! God dammit Amazon, I just bought a crock pot! You know that! Why do you still serve me ads about crock pots!? The whole point of the surveillance state is to make use of all that info you have on me - frankly, it's insulting. I demand a better big brother!

It drives me nuts when I log in to Amazon and it keeps showing me all these parts for building a new PC.

Damnit Amazon I just built one! How many processors do you think a single gaming PC can fit?!

I've often wondered why they do this. I see more variations of items I just purchased rather than accompanying/related items.

it works for a lot of cases, e.g. buy a book and get other books recommended, not so well for rarer purchases like laptops or phones.

I still get recommendations for the same book I just bought, but in different editions/formats. Seems like it could be improved some more.

But be careful of the 'Perfect Partner / frequently bought with...' recommendations, which Amazon themselves disavow as being in any way compatible:

If two items listed as Perfect Partner / Frequently Bought Together are not compatible, Amazon.co.uk are unable to accept the return of either item for this reason.

That's because it is just doing item-based collaborative filtering, right? It works beautifully other times. I bought chopsticks once and it showed me not more chopsticks, but other accessories for sushi, etc.