[copied in from a comment for further discussion re: the NSA]
We can skip the word "evil" and just take examples of what citizens might want to know.
While apparently the post office has been scanning envelopes a long time, the use of OCR means they have a huge searchable database of From, To and Date. The recent improvements in data analytics & Big Data means it's trivial to compile that into categories - especially if you link it to Facebook and collected phone metadata - who your friends are, where you work, where you bank, who you call, what organizations you're involved with. Depending on who's sending you mail, they can figure out if you're having credit problems, health issues, seeing a psychiatrist, getting a divorce.