You still need ammunition for 3D printed weapons, even though i am very much certain, that the individual ammunition is not tracked or has a serial number.

For 3D printers it is relevant information, so police can have a look into who has one, if they believe a printed weapon to be used, or find one. This in itself is not a criteria that warrants a search, but if Neonazi A is murdering someone with a printed weapon and Neonazi B from his Naziclub has a printer, that is a good place to start looking. Or like @ProcurementCat said, it creates a context in which people buy black market, which involves more fuck ups.

Not to mention you can actually quite easily build it yourself.

That is relative. It requires much more effort to build one, than to buy one. Especially if you need parts with a tight tolerance, like for a gun that can actually shoot things.

In the same wake it is very much possible to make all the precursor chemicals to make illegal drugs, still regulating some key precursor chemicals keeps most hillbillys from making drugs in their backyard.