From Jay Leno's Garage

[gtx6970]

The plastic one can be used to make a mold for your new part.

I agree , But if your scanning an original part why can't it be used to scan  / copy to make an original part from

[dodgecharger-fan]

With the plastic, you can reshape/refine the model before sending it off to the casting process.

For example, the crack in that original piece is exactly replicated in the plastic model. It's easier to fix that in plastic, get it to the shape and finish that you need, THEN use it to create the mold for the metal casting.... and if you mess up the plastic piece, just print off another and start again.

And the fact that the piece will be cast is another reason - if it were going to be machined, you might be able to go right to making the piece and skip the plastic step. Even so, it might be good to print off a plastic piece to make sure the scan went well and the piece is exactly what you need before you start carving in to a chink of metal.

To cast, you need a mold and you might not want to use an original to pull a mold - in fact every casting process I've ever seen always starts with making a replica of the original that is used to make the mold for the final casting.

[dodgecharger-fan]

Wonder if it could replicate itself? 

These do: www.rrrf.org

[wayfast1500]

Id like to try to put a mirror in front of it.

[Tilar]

That might be interesting.