Tungsten carbide dies are used in cold forming and cold heading manufacturing practices. Basically, the very, very hard material (tungsten carbide) used to make carbide dies allows them to take a rigorous beating and this is crucial. In cold forming cold metal is pounded into the die, forming the desired shape. This impact is so powerful that the metal slug actually becomes liquid as it is forced into the die. Furthermore the metal used is often steal, so something even harder is required when using cold forming manufacturing techniques. Tungsten carbide is the second hardest material known to man, just behind diamonds and diamond dies would be ridiculously expensive!
Cold forming and cold heading is a very productive way to make parts. In fact, parts can be made every second using this method while machining parts or using a molten metal and a mold takes much longer. An added bonus is that cold formed parts have greater strength because the grain of the metal is reshaped, as opposed to cut into when machined.
Basically, if you want to make a lot of something quickly, then you want to use carbide dies and cold forming.