What makes 3-D printing such a potential gamechanger is that it can "deliver" cheap, customized items on command. Once we get good at things like printing mechanical arms, it will be trivial to customize each arm to the person that needs it. Parts and devices that are only needed sporadically can be printed at will, and people in hard-to-reach places will not need to spend time and money on shipping each time they need to order something.
And that's just the everyday stuff. Printable food, tissue and circuitry is in the future at some point, and each could be revolutionary.