The value is "You'll never lose this note, idea, or tool ever again in your entire life. Even if it goes offline, you'll still have a starting point for remembering that-thing-you-saw-years-ago-and-want-again."
There's a certain phenomenon in the gamedev industry that was discovered a few years ago. As you price something near zero, your sales increase more than linearly. You'd think that if you drop the price of a game from $50 to $5, you'd get 10x the sales. But in fact, you end up with much more. I wonder if something similar happens for Pinboard.