The concept of a shotgun can be used to create DIY weapons, which are nowhere near as efficient as the real thing. However, it is sufficient to make a very dangerous DIY device that are usually single shot. For example, launching a hard object through a tube (barrel) using a chemical propellant in a confined space is enough to turn that object into a dangerous projectile.
There are many reasons why someone may want to do this. Many people are naturally fascinated by weapons, and destroying things can be quite fun. These people are the hobbyists, who have no ill intent. The other side are people looking to use it for malicious purposes.