Let’s get it out of the way right now: you don’t need a bone folder. You could use a butter knife to do more or less the same thing most of the time, but once you have a bone folder, you won’t. Nor will you use the bone folder to butter toast. The knife will be in the kitchen, where it belongs, and the folder will be in the office. It will be right there whenever you need it. 
Reaching for the bone folder makes you think of a Victorian lady or gentleman writing many letters in longhand, by gaslight, and reaching for the bone folder after each one to crease it neatly before stuffing into the envelope. You can do that, too, but I’m guessing not too many of us still write a lot of letters to people who might care about the neatness of the crease.
I got mine when I was learning the rudiments of making pop-ups. Inspired by Robert Sabuda, I tried it myself, and one of the things I found out was the importance of the bone folder in the art of mechanical paper engineering. The point on the one end is for scoring before you make a fold, and the blunt end is for creasing, flattening and burnishing. You use it all the time.
But let’s say you’re not planning on writing your own pop-up books. Why would you need a bone folder in a 21st-century paperless office, with scanners and laser printers? Well, we all know what happened to the paperless office: it’s the one we drive to in our flying cars. You’d be surprised how often you need to fold stuff, so if you have to do it, do it well. They are handy for children’s crafts, too; anything that helps children do things more neatly is always worthwhile. Finally, when packing things in boxes to ship via UPS (a very 21st century task), you can burnish the tape down tighter than ever before and really get that box sealed.
So put the butter knife back in the kitchen and pick up a bone folder from Amazon or your local art supply store. They’re cheap, too.
