Obviously to have an unregistered gun?

To get it through security somewhere with metal detectors. That's probably the only reason to specifically fear a 3D-printed gun in a nation full of proper guns.

Of course, 3D printed plastic ammo isn't likely to be very effective.

(Maybe they're worried that before long, 3D printing with metal will almost as easy and affordable as plastic 3D printing is now, and people will be printing off entire arsenals of very effective firearms?)

Is this a real question? Legally buying guns in the US come with registration of serial numbers, names, and addresses. Printing a gun does not. Printing a gun also does not need to wait for a multi-day delay from a background check. Depending on the printer, it could just take multiple days to print.

Asking why someone would want to do this is just not trying very hard in the conversation is actually pretty myopic.