Security is the main sales argument. You can share an entry if you want to but otherwise it's completely unsocial. Passwords are encrypted and each entry can even have its own password. If you decide to pay a $19 annual subscription you can get the Penzu Pro edition with even stricter encryption. It's basically blogging for an audience of one.
I like the idea though I'm so used to social tools that I can't really think of anything to write that I don't mind sharing. However if you're making notes on research that you want to keep safe or are busy gathering material for, say, a book it could be ideal in that your material is always accessible rather than being tied to one computer or on paper in a drawer. If you're travelling and need to store notes and reflections safely you can acces this from any computer.
You can, of course, keep your private notes on Google Docs or even on Facebook as long as you lock down the social settings so that no-one else can access them but this is the first tool I've seen that has total privacy as its unique selling point. I might just give it a try.