As a regular reader of our site might know, Republic of Langrangia are strongly in favour of 3D printers (see here and here). Recently, this idea has become mainstream. On space.com you can read the following article: 3D Printing Could Aid Deep-Space Exploration, NASA Chief Says.
The basic argument for 3D-printing in Space are simple: our Solar system is rich in all kinds of resources. Through in situ resource utilization the input for 3D printers can be made from asteroidal material (or if necessary from Lunar or Martian material). The blue prints of the needed objects can be stored electronically on board of a Space colony or can be transmitted from Earth. This would significantly reduce the launch of all kind of supplies.
3D printing will also reduce the number of people needed for Space industries, because a 3D printer can print a whole variety of things. And in principle, we have to launch only one 3D printer, since this machines should be able to reproduce themselves.