Yes, indeed, as an individual nobody cares about you. However you are a part of general census and by spying on you and everyone around you they can predict what you are thinking, what you are craving, what political stance you hold, what are the risks of you being involved in some future problems for them etc. You can go wider and wider to a city, to a state and finally to a country. Then, for example China can really see what american citizens are thinking in general and this is valuable information.
Also, second reason why you are completely missing the point. Let's say you are one of those "I don't care if they spy on me, let them record me on my toilet." - Now your kids are involved in some political activity and you did not pay your taxes for one month 7 years ago. Government wants to suppress your kids political activity and just tell them either they stop or their father (you) go to jail. At least this is how it works in totalitarian countries.