What are they for?
They are great for making one-of-a-kind items and low volume production parts--prototypes and experimental designs. Among other things, a 3-D printed part can be used as the master to make molds for sand-castings. With the right (professional) equipment, it's faster and less expensive than making metal tools for high-colume production in a machine shop. For limited quantities the final parts can be of equal quality (until the plastic master wears down--which is why high-volume production tools aren't made of plastic).