Additive manufacturing technologies such as 3D printing are having a big impact in many areas as they are incredibly useful. An analogy is building a house, you could carve one from a solid lump of rock, or you could make one from bricks stuck together with mortar.
3D printing is built up in layers, much like courses of brick, allowing flexible design with minimal use of material. Plastic is the most common material used in 3D printing. Small plastic 3D printers are available from a few hundred pounds. Other materials such as metal, composites and even concrete can also be 3D printed.
Because additive manufacturing is usually computer controlled it allows a very flexible process, changes in design being made immediately in production. This is why 3D printing is so useful for design studios, prototyping and bespoke manufacture. One inexpensive machine can manufacture many different parts that would require many traditional processes.
One issue with 3D printing is speed. The time taken to make a small plastic toy in a 3D printer can be measured in hours. An industrial injection moulding machine could make hundreds of the same toy in seconds. Clearly 3D printing is obviously not always the best choice.
So it is the flexibility that is key to additive manufacturing. In this respect they are some of the most useful tools available, much like robots.