There are a great many tweets, blog posts, articles, papers and books on this topic. Here are summaries of three of them in order of accessibility. First some quips in response to the classic question (from Schneier, see below for why these aren't the right answer though).
- "If I'm not doing anything wrong, then you have no cause to watch me."
- "Because the government gets to define what's wrong, and they keep changing the definition."
- "Because you might do something wrong with my information."
Falkvinge has a good brief blog post on this topic, further summarised here:
The rules might change. Do you know what the next government's policies and laws are going to be? You know, the government voted in by that relatively small voting group that you fundamentally disagree with but managed to form part of a coalition government?
It's not you who decide if you have something to fear. It's automated surveillance software. People will start to behave not just based on what is legal or right, but based on a desire to avoid being flagged.
Laws must be broken for society to progress. A controversial statement when you first come across it, but self-evident when you think about it (the legality of homosexuality is a good example). Lack of privacy prevents this.
Privacy is a basic human need. See @Andrew and @Rory's answers for some obvious examples, and the paper below for a deeper understanding.
Schneier, predictably, has a (short) essay on this topic. He argues (like Solove - see below - but more accessible) that the question is wrong to frame privacy about hiding wrongs. He prefers to frame the debate as liberty vs tyranny.
On abuse of data:
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.
If you're looking for more in depth treatment, try Daniel Solove's paper, who argues that there is a fundamental problem with the focus of the question on privacy as just being about hiding. Here's an online version of the arguments from the same author.
On this topic:
The harms consist of those created by bureaucracies—indifference, errors,
abuses, frustration, and lack of transparency and accountability.
On why privacy should be a default for information, not just on information you consider to be directly sensitive:
Aggregation, however, means that by combining
pieces of information we might not care to conceal, the government can
glean information about us that we might really want to conceal.
The problems of surveillance by a government in particular
it creates a power imbalance between individuals
and the government ... This issue is not about whether the information gathered is something people want to hide, but rather about the power and the
structure of government.
One of the problems found in countering this argument is that it doesn't sell very well:
At the end of the day, privacy is not a horror movie, and
demanding more palpable harms [caused by privacy violations] will be difficult in many cases. Yet there is still a harm worth addressing, even if it is not sensationalistic.
So, the one liners at the top of this answer are weak responses, as in responding in that manner you've already fallen into the trap of focussing solely on the aspect of privacy that is about hiding wrongs or embarrassing information.
You may also want to check out Lessons From The Identity Trail, a book formed from a fantastic collection of papers, now freely available, that were the output for a multimillion dollar study on privacy, identity and anonymity.