While the low end 3D printers (like the ones that I have) cannot print arbitrary shapes (they need a fairly broad base to stick to the printer bed during printing), they can print shapes that would challenge normal manufacturing techniques.

One example may be found here. This is a ring gear with 5 internal gears all surrounding a single central gear.  These gears are unusual in that the teeth are herringbone, so not only can’t this piece be disassembled, it could never have been assembled in the first place using traditional manufacturing techniques.

 

 

Here are some herringbone gears on their own so you can see how they are shaped:

 

 

Another example is this fan shroud. The fan is positioned to one side and blows air into a plenum.  The base of the plenum has a circular hole through which the air exits.  The point of this design is to blow fan around the printer extruder hot end, but not on the hot end.  The shape is complex and could never be injection molded without breaking it into multiple pieces, and then assembling those pieces.  With a 3D printer, you just print the part: it does not need assembly.

 

 

 

 

Here I show what the part looks like internally, after I sawed it in half:

 

 

Two shrouds in the process of being printed: