The main reason for using tungsten in fly fishing is its incredible density. The name tungsten means heavy stone from the Swedish (tung+sten ) tung, heavy (from Old Norse thungr) and sten, stone (from Old Norse steinn).
It has a density of 19.3 g/cm3 - the same as Gold, but much heavier than Lead at 11.3 g/cm3 and double that of Brass 9.8 g/cm3.
Since sink rate in water depends on the density and profile of the object, if you want a fly to sink fast, add tungsten! It is dense enough that you can use a smaller fly and still have it sink to where the fish are holding and feeding.
Don't confuse much cheaper brass beads with these beads, brass beads are only about half the weight for a given size of bead. Another plus for tungsten is its low toxicity, it does not appear to harm to animals if ingested.