When I was younger and at home for winter or summer breaks, I used to watch Martha Stewart's show in the mornings. Yes, I am a true craft nerd! I think it aired around 10 am, after the morning news shows. Whenever she'd make craft projects, she used a bone folder. I'd watch her create in awe, but I also scoffed at the idea of using a bone folder. Why would I need a specific tool just to fold paper?
Well, now that I have a bone folder, I don't ever want to be without it! Mine is by the brand EK Success, as pictured above. I got it on sale at JoAnn, if you're wondering. I make a lot of cards and when working with cardstock, it can be hard to get a clean fold. Now that I use a bone folder, I can tell when someone has just used their fingers: the cardstock is all wrinkly around the fold, and the fold is not as tight. Even if you don't notice this, you will notice if a card is not standing straight, or if it isn't lying flat. A bone folder helps create a tight crease which in turn helps with a card's "posture," if you will.
You can also use the tip of the folder to score along piece of cardstock. This is great for not only making cards, but making paper boxes, etc. I used to use a ruler for folding, but it would often leave lines or marks in my paper since it wasn't completely smooth. The plastic of the folder (and no, it isn't actually made of bone!) has a nice weight to it, is very smooth and also appears to be sturdy. It doesn't leave unnecessary marks or creases on my paper, and so far, I have nothing negative to say about it.