Cost down would be an important factor for the profit to the companies, especially for the products would be made in bulk quantity such as the case of MP3 players. Since it's all the same products with less concern of precise, the mold tooling would be a great idea to generate the products with less cost.
The most common case should be the various kind of cases for mobile phone, it could have the different logo or drawing in the back, but the positions of the holes for chargers, memory cards and headphones would be the same. In this situation, you can produce the case for latest model in a few time via mold tooling.
If you are trying to make the special or limited products in few quantity, maybe the extra cost of making a new mold would be a bad idea. However, if you are suppliers of general products, the mold tooling could be quite practical and save more cost than other methods.