How so? That is really non-obvious to me. If site A associates user X with IDs 1,2, and 3 over three weeks, how does that help site B that only sees the IDs? Wh

How so? That is really non-obvious to me. If site A associates user X with IDs 1,2, and 3 over three weeks, how does that help site B that only sees the IDs? When B sees ID 3, without any further unique identifier, they won't know that the same device came with ID 1 and 2 beforehand. If site B also had an account and both sites would work together, they could simply compare the email address.