I think it goes without saying you can build things however you like.
I'd recommend owning a 3D printer because they're just amazingly useful tools and I'm trying to be helpful, but I have zero interest in arguing with someone who doesn't want to use one. Go your own way.
But to be clear, this isn't about making things to get a job, making things to sell, or making multiple versions of your game. When I said making 15 of something the same I don't mean making 15 playfields. Though of course that's always an option. Or maybe you and a partner are making a game together and building two side by side and sharing parts. But just a single solo build has so many use cases.
Maybe you want a little clip to hold your insert lights the perfect distance away because they're hot spotting too much. You can work on a design, printing something, trying it, adjusting and making new ones until you're happy with the distance and how they fit in etc. Then print enough for all your inserts. They'll all have your design you worked out duplicated perfectly and easily and you get get your playfield looking like you wanted.
Maybe you're printing diffusers out of transparent filament. Print the whole clip and the diffuser out of the same material.
If you want to just screw your little PCBs straight into the playfield instead that's fine. If you want to hand cut spacers out of wood with a coping saw that's fine too. Just like you can hand drill a playfield, rout it with templates, or use a CNC.
3D printers are just another tool. But for something like making pinball where there are a million different little use cases for custom parts it can really shine. Print up a diverter arm. Print brackets to hold ball guides. Print clips to hold habit rails while you design your wireforms. Make mounts for your in-line playfield segmented displays.