In many ways the metadata is more telling than the actual data
An account comes online every day from 9am and offline at 6pm, five days a week that tells one story.
Two phones travelling in opposite directions towards a hotel every Friday afternoon and powering off shortly before reaching the destination, and then powering back about an hour later when the phones are heading away from the hotel tells another story. Almost every application installed on a mobile phone these days has the potentially to capture and upload that kind of metadata. Historically (pre-smartphone apps) only the mobile telephone operators could track your cell location, and they did that and and still do record that information and more to monitor the existing, and improve the future, picocell/nanocell/microcell/base stations coverage. Historically they would throw it away after use, now it is stored permanently because it is so cheap. Now this metadata is available to the telco provider, the operating system provider, the phone hardware manufacturer and all the individual application developers.
Long term metadata collection can leak far more information than data normally does.
Cloud providers with the amount of metadata that they are harvesting can tell when you are eating, using the toilet, or when your normal routine had changed even by slightest amount.