Speaking of personal tracking for the purpose of creating better products, one of my favorite examples is Google Maps’ Timeline feature. I have discovered its true potential only after switching to Android, as I think on iOS the Google Maps app does not have permanent access to the device location. In a sense, the Timeline is a nicer, user-friendly representation of Location History, which Google happily collects in the background from every Android user.
You may of course turn this feature off or selectively delete portions of this history. Personally though, I love to be able to scroll back in time and see where I was months or years ago (well, three years ago at the most, as little was saved while I was using an iPhone): the place, when I arrived and left, if I was walking, in public transportation or in a car. It is a very convenient way to log my travels and recount them later, in case I would like to revisit those places. For a good portion of time, I also used this to fill my timesheet at the office, as my company’s badge did not correctly register the time I arrived and left the building. For photographers, daily tracks can be exported in a standardized format, which you can then use in Lightroom to add location to photos captured on that day. I use a dedicated app for that, but it’s good to have a backup option in case the app fails or gets discontinued at some point in the future.