You're filling out a few fields with fake data, but filling thousands of "fields" daily, for real. Those come from your behaviour: what sites you visit, when, with what frequency, what do you click, what do you install, who are your contacts, when do you sleep, when do you move, how often you send messages... I can not even imagine how long the list must be from all the things you can extract from someone's internet + mobile behaviour.
Edit: and that's what I find most troubling: that someone can know much more about me than myself. Things that you can only realize from looking at the data over time, things that are invisible when experienced on real time.
The part you find troubling is likely the most exciting part---once harnessed and turned to an individual's utility. "Something that knows more about me than myself" was the dream of the 'personal digital assistant' craze in the '90s, but the technology wasn't there yet. It's here now, but it leverages network advantages and scalability to work its magic.
I imagine the synthesis of these forces in the future is technologies that aggregate, but that the indivudal can buy into. Do you want help deciding what to wear based on the weather or time of day, beyond what your assistant can do for you? Then there's a collective data channel you can opt-in to joining, and you'll trade some of your personal private information for access to a digital "collective unconscious" of aggregate data. I liken it to the 23andme approach of collecting genetic profiles.
There is a difference between "knowing more about you" and "helping you find out something new about you". The entity that knows more about everyone than they themselves do concentrates power. A tool that you use to find out something new about yourself, only for yourself, does not.
Edit: and that's what I find most troubling: that someone can know much more about me than myself. Things that you can only realize from looking at the data over time, things that are invisible when experienced on real time.