Why was her diary secret? Well, because she wanted it so. And that should be enough.

When it comes to information that any given person regards as intimate about oneself, that should always be enough. It doesn't matter that someone else might peruse that information and say, "there is nothing special here." Why? Because it is not that person's judgment to make. There may be nothing special about my exact bank balance or net worth or state of my health or sex life or political or religious affiliations (or lack of affiliations) or any other information that I may consider private but that doesn't mean I want that information broadcast to the world only to invite identity thieves, malicious third-parties, political enemies, or anybody else who might bear a grudge or harbor an animus to have a field day with it. But they are just "facts", you might say. No, in the wrong hands, they are ammunition by which to hurt you if people want and, even if the facts themselves are innocent enough, people have a ready capacity as may suit their whims or prejudices or any other ulterior motives they may have to do you wrong and what may appear as innocent "facts" to one person can easily be transmuted into a vile weapon that can cost you your job, your important relationship, or any other of many things that might be the subject of someone's jealousy or other animus toward you (a lifetime of courtroom experience has certainly shown me how easy it is for a motivated adversary to take otherwise innocent facts and twist them into malicious aspersions if they really want to). What is more, even if nobody had any interest whatever in harming me through misuse of that information, it is part of my essential humanity that I can separate that which I regard as intimate (to me) from that which is made freely available for public consumption. There is a reason why the word "vulgar" developed negative connotations over the centuries: it originally meant nothing more than "belonging to the crowd." Well, crowds can trample on things you might regard as precious and it should be your choice and no one else's whether you want to open up important parts of your life for public consumption.

If I happen not to want to keep any part of my life secret, well, that is fine too. That is a choice every person can make for himself so long as he doesn't go about over-sanctimoniously proclaiming that others must do the same or harming others about him by revealing things that they consider intimate as he opens the book of his own life to the world.

It is the same in the business world. One can open-source his own works as a matter of commitment to the idea that all information ought to be free or for any other reason but that doesn't mean the law ought to abrogate protections for proprietary, trade secret information that most businesses need to keep confidential information as a matter of competitive advantage. If I am a broker who depends for his livelihood in serving a customer base that it took years to develop, I would be rightly upset if someone came in and simply handed all my customer information over to my competitors. So too would a development team that has invested huge amounts of money and time into a development effort that gives them a significant competitive advantage over others and whose business model turns on keeping that advantage to themselves exclusively. So too would most any company management if its confidential business plans for winning key markets suddenly got broadcast publicly over the web. Examples of this type can be multiplied endlessly and really are self-evident to anyone who has had much in the way of real-world business experience.

Again, any private business is free to make a contrary judgment and to open itself up at every level so that it maintains no private or secret information whatever. That is their choice. But, if I want to keep things secret in my business, no one should be able to force me to do otherwise or to try to shame me into believing that I am doing something wrong.

Laws and public policy cannot make these choices for us as individual actors but it is essential that they set up a structure to protect those who would seek to keep their confidential information private. How and to what degree that happens in practical execution can be a complex topic in our technological age but the abiding principle, to me, is very clear: privacy is valuable in any society and laws should be shaped accordingly.

So, why, then, is this secret? Because the person it most affects wants it that way and the rest of us should respect that person's wishes to keep it so.