What is 3D printing good for in plastics?
Low stress, non structural pieces. Where does 3-D have a great application? Mock up.

Reality is, if sticking in the plastics family, there is no substitute for injection molding. If you need more structure, inject into the mold glass filaments to increase tensile strength. Why not make the whole thing out of glass? Weight and no ductility aka fracture resistance. Better strength to weight? Carbon fiber. Composites if trying to optimize mechanical properties like modulus of elasticity, weight and yield strength make a lot of sense. Nobody has figured out how to do this on a printer yet.

No doubt 3D printers will evolve...but bi-material aka composite printing will be very tricky technically for a balance of weight and strength in different planes.

Generally when a product is developed in CAD...a virtual 3D model, many times engineers will want some touchy-feely parts...or design studio will want to make up a representation of a topographical surface to evaluate aesthetics. This is done in clay at the design studio initially and then the surface is scanned to create the math surface. For more mundane function parts, stereo lithography is popular to create non structural models of proposed parts...either to scale or reduced size. Latter is where 3D printers will shine and replace SLA models. The public...maybe the OP falls in this camp, blindly believe that many great parts...a bike fork (shudder...lol) can be made on a 3D printer. No doubt, average Joe will buy a 3D printer and even try this. Kind of like the first guys that developed air planes who didn't quite have the right L/D equation worked out who ran their plane off the cliff in an attempt to fly. Never heard from those guys again. The Wright brothers who btw owned a bicycle shop, were a little smarter than that.