Since society isn’t intelligently designed any more than the human individual is, it’s not really the case that privacy, or the familiar injunction against “unreasonable” search and seizure, is “for” any one specific purpose.
Of course humans since the earliest days kept secrets, both to influence opinion and to show that they could (a taboo that is routinely violated can persist because it helps people demonstrate their ability to keep secrets and thus convince others to trust them with their secrets).
Later, people who had won some measure of influence over their governments became extremely interested in using that influence to limit “unreasonable searches and seizures”. This was to preserve their own secrets in noncriminal matters, and also because searches and seizures were extremely intrusive and inconvenient affairs – the authorities barged in, rifled through your posessions and papers, and took away anything that looked interesting, all while waving swords or guns at you. People began to object particularly when these things were done without any reason to expect the investigation to actually uncover criminal activity – that’s a pretty rotten thing to do to someone that’s almost certainly innocent of any wrongdoing. And, wherever people were able to influence their governments, they were quick to place limits on the use of searches and seizures, to require some sort of probable cause, and so on.
This had the pleasant side effect of making it difficult to enforce laws on matters that didn’t come to the attention of the authorities – matters where no one turned up missing or dead and no one complained to the authorities. Personal matters, that is, and private activities between “consenting adults”.
This last benefit seems to have become predominant – even though technology allows the authorities to collect many sorts of information without the target even knowing about it (thus rendering moot earlier objections to the intrusiveness and inconvenience of arbitrary searches), the fact that limits on the gathering of information still selectively weakens the government’s power to enforce laws on personal matters means that those limits are still a useful and important feature of a free society, or at least one that aspires to stay that way.
Unfortunately, some private activities now have the potential to severely weaken public order by getting a lot of people killed at once. Thus, those selective limits now aren’t so selective; instead of only suppressing the enforcement of laws that have at most a tenuous relationship to the maintenance of public order, our traditional limits on intelligence gathering suppresses the enforcement of certain laws that are absolutely indispensible to the protection of life and property.
(Well, not absolutely indispensible. There are alternatives, but those involve drastic changes such as the universal adoption of personal aircraft, the obsolescence of cities, and a more uniform population density throughout the civilized world. Personal nuclear reactors to lessen the dependence of large groups of people on fragile centralized infrastructure of several sorts would also be helpful. But our culture places a high priority on preventing natural selection in the human species, so there’s a lot of resistance to those alternatives).
Which means the old workarounds aren’t going to work so well anymore. New workarounds are needed. One way out of this dilemma is to allow the government to collect any information it wants, but only for stopping terrorists in their tracks; anything they happen to find out about a non-terrorist’s activities is quietly forgotten and does not become available to prosecutors, and the very existence of this setup is kept as quiet as possible. That seems to be the current workaround, but it’s vulnerable to “mission creep” – stuff like drug trafficking, money laundering, and child porn have a way of getting tacked on to the list of things that the unlimited intelligence gatherers are tasked with thwarting (just start calling them “global threats”, and voila – they’re fair game), and there’s no telling what’ll end up on that list down the road, especially after people have gotten used to the whole setup.
Another way around the problem might be to universally allow unlimited non-intrustive intelligence gathering, and devote lots of resources to make sure that every infraction of the law is prosecuted to the fullest extent. Do it up front and all at once with as much fanfare as possible and let the people decide if those laws that are suddenly all too enforceable are really worth keeping. Couple this change with a large scale sunsetting of existing law, so elected officials will have to campaign and vote for a law rather than simply neglect to vote for or sponsor a bill for repealing it. Throw in a regular sunset of new law, so that they’ll have to vote for it again after seeing the practical effects of those laws when they’re actually enforced.
We’d end up with either a stable, well-defended, free society or a harsh tyranny. But if the people are disposed to support tyranny in that setup, those same people will support it by degrees in the course we are currently on, and nothing short of a takeover by a liberal (in the non-leftist sense of the word) long-lived king will ultimately stop them. (Good luck finding one!)