Bruce Schneier provides a basic answer to a very common question:

The most common retort against privacy advocates — by those in favor of ID checks, cameras, databases, data mining and other wholesale surveillance measures — is this line: "If you aren’t doing anything wrong, what do you have to hide?"

The Eternal Value of Privacy By Bruce Schneier, 02:00 AM May, 18, 2006 Wired

After rehearsing a few true yet not deep enough comebacks that people sometimes use, he gets to the heart of the matter:

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.

Not to mention what about when you sometimes speed at night on that empty road, or forget to fasten your seatbelt, or wrote something on the net uncomplimentary about a political candidate that your boss favors, or….

How do you know you haven’t done anything wrong, in somebody’s eyes?