Projectiles are a part of human nature. We've always thrown spears, rocks, etc -- firearms are just an extension of our better understanding of the world. I know of barely anything else that uses explosive charges that is as widely applicable to the general public. Roofing nail guns? But that's such a niche subject, it's not something people are really worried about trying to make with 3D printing. Believe me, if I had a better engineering challenge for 3D printing, I'd be suggesting it. But nothing quite hits like containing an explosive charge, and utilizing the energy in a way that performs work without destroying itself.