Everywhere you go, you are being followed. Not by some creep in a raincoat, but by the advertisers wanting to sell you things.
The more advertisers know about you — where you go, which shops you visit, and what purchases you make — the more they can profile you, understand your tastes, your hobbies and interests, and use that information to target you with ads. You can thank the phone in your pocket — the apps on it, to be more accurate — that invisibly spits out gobs of data about you as you go about your day.
Your location, chief among the data, is by far the most revealing.
Apps, just like websites, are filled with trackers that send your real-time location to data