[ForHackernews]

They know you're wealthy enough to travel to Europe. They know you might buy airplane tickets. They can show you ads for other international destinations.

If nothing else, they can sell ads against you for "Category: Travelled to X place within the last Y years".

[netsharc]

Thought experiment: your phone could know you liked a particular tourist spot because you spent a lot of time there, and later on when you upload pictures from your DSLR to Google Photos, the system will notice that there are a lot of photos that are timestamped within the time range when you were at that tourist spot.

And data mining can tell that people who like that tourist spot A in country X also liked tourist spot B in city G in country Y a lot, so, tourism board of city G/country Y, here's a good target for your ad.

And not just tourist stuff. Data mining might even tell they like a particular brand of, I don't know, yoghurt, so let's sell some of that yoghurt!