There are folks in the RC car world that are 3D printing parts for vintage RC Cars that are not supported by the manufacturer anymore.
All kinds of things like transmission cases, suspension mounts, control arms, steering arms.
There was one project that built a replica of the 1991 World champion RC10 that was never released to the public. It's made from a mix of 3D parts, stock parts and milled carbon fiber plate.
As for useful 3d printed items.. How about a rocket engine using a sintered metal process.
You can also 3d print wax for investment casting a metal final product. (you can do this at home)
Lots of ways to use a 3D printer if you use your imagination.