So a 3d printer’s (person not machine) favourite words being answered as to why you want something printed. “Hard to come by and expensive.”
The other would be “because I can” 
Or why did you choose a feature (ie harringbone gears) they will cite some small benefit to the design(0 backlash 0 thrust) then follow it up with “and it cannot be machined or injection molded”.
3d printing is useful for making 1 off designs, small toys you would find in a $0.25 machine, and learning / iterating on a design.
Yep, “can’t make it any other way” is my favorite.
@Ryan_Carlyle and 99% of the time you can’t make it any other way because of a tiny design choice. 20% of the time you could easily make it another way but the designer wanted to print it and was looking for justification.
@Camerin_hahn Completely agree
Cells bowls and voronoi prints and such are lovely examples of unnecessary things that are awesome and can only be 3d printed. I also really like print-in-place gadgets.
@Ryan_Carlyle I love them as well, but really you only need a 3d printer if you need a 3d printer.
@Camerin_hahn You do realize that’s a tautology, right?
@Whosa_whatsis yes. That was the intent. I am saying that if you are wondering do I need a printer the answer is probably no.
Of home manufacturing processes, 3D printing takes the least amount of physical space and tooling with the most possible output, and digital design is the only process that comes with an undo button.
If you want to keep a shop full of machinery and tools and safety gear for woodworking or metalworking, have at it! If you’re not renting anymore and your home has space for a kiln and clay’s your thing, go nuts! Do it all! Do none of it! It’s all good! =D
I don’t think I’d say “this can’t be made any other way,” but I’d probably say “I can’t make this object any other way and nobody else is going to do it for me.”