Two proverbs say it best: Quis custodiet custodes ipsos? ("Who watches the watchers?") and "Absolute power corrupts absolutely."
Cardinal Richelieu understood the value of surveillance when he famously said, "If one would give me six lines written by the hand of the most honest man, I would find something in them to have him hanged." Watch someone long enough, and you'll find something to arrest -- or just blackmail -- with. Privacy is important because without it, surveillance information will be abused: to peep, to sell to marketers and to spy on political enemies -- whoever they happen to be at the time.
Privacy protects us from abuses by those in power, even if we're doing nothing wrong at the time of surveillance.
We do nothing wrong when we make love or go to the bathroom. We are not deliberately hiding anything when we seek out private places for reflection or conversation. We keep private journals, sing in the privacy of the shower, and write letters to secret lovers and then burn them. Privacy is a basic human need.