They are capable of creating items which would be impossible or very difficult by reductive manufacturing techniques for example internal hollows, variable infills to strike the right balance between weight and strength, variable wall thickness etc. You can manufacture every part in a design for optimum performance without any concessions to the practicalities of the manufacturing process. As tom osselton pointed out, it would take lot more time and human effort (programming, setting up plus or minus manual machining) to make complex parts than printing them out. Boeing have started using 3d printed parts on some of their commercial aircraft and airbus are looking at doing the same. They are companies that spend an awful lot of effort in optimising their design and manufacturing process.
On a more grassroots level, 3D printing offers people with limited skills and abilities the chance to create whatever they can imagine. I don't have an engineering background, my machining abilities are rudimentary and CAD skills are nonexistent but this doesn't stop me from creating great quality stuff with software created for dummies (like tinkercad and 123design), and printing them out.