> Amazon (or whoever) showing you the same stuff you just bought is kind of a separate issue, because it obviously isn't useful to anyone (including Amazon) and surely is so easy to avoid I find it bizarre that it still happens in this day and age.

Isn't the point of those, that in the end, the likelihood of you buying something again just recently bought might be tiny, but is probably still bigger than showing the same add to a random other person?

Essentially you're right.

The effect is quite strong.

Humans are stupid and fail to make logical decisions, instead they reinforce and justify their previous ones.

Showing you the product you purchased is the best way to get you to transfer your purchase decision to purchase additional other products.