Because it's social media, and such free systems are built around collecting and selling user information for all sorts of reasons. Mobile users especially, because it's easier to get more full pictures and data sets on people who use a mobile app. Truth won't be any different, will say they are doing so to put "relevant" ads that more fit the user's interests. It's how free systems on the internet like this work, and as I said, this site and app will be no different. It's the main way such sites bring in money and make profits.