That is a great question. As a certified technician for StrataSys 3D printers, I have seen many different implementations for 3D Printers in molding companies. These great examples come to mind.
1. For iron cast moldings, one of my customers used the 3D printer to create a casting for their sand blocks. They then use that sand block to cast their iron moldings. It was a very clever implentation for them and quicker and easier to rapidly prototype a plastic mold for their sand casting then it was to machine one. This is especially true for any changes. Like for all of my examples, the 3D printer allows for very quick model changes.
2. For transparent container moldings. One of my customers creates plastic moldings for retail products. For example, think of a toy car hanging on the shelf. That clear plastic molding that holds the car to the cardboard backing can be quickly created as a negative mold in the 3D Printer and then can be used to vacuum mold the production plastic piece. The use of vacuum molding is very quick and efficient with a 3D Printer.
3. For paper packing molds. When you ship fragile items, usually you will see that the glass item (wine, for example) will be in a paper pulp mold that fits the bottle and protects it from breaking. This paper pulp molding works perfectly with a 3D printer that allows water to be sucked thru the 3D model leaving the paper pulp mold to collect in the shape of the model. When dried, you have your packing material. Again, very quick and efficient use of water molding.
4. For kitchen products. Another customer used the 3D printer to prototype all of their cookie cutters, and specialty cake pans. Once again, this is a quick way to test the molding before going into production.
5. Finally, numerous customers will use the 3D printer to create the fascades of their production equipment before casting their final parts. Whether it be for electrical housings, appliances, remote controls, or just any general look-and-feel cases, the 3D printer allows them to see and hold their finished product and make ergonomic changes before production.
I hope that these real-world examples help to answer your question.