The heads of the nymphs play different roles during fishing, they attract fish by their flashes and colour, thanks to their weight they enable the nymphs to sink quickly to fishing zone and keep flies in the right position which is most effective.

Tungsten has a specific weight of 18 kg/dm³, which is nearly twice more than the lead.
Therefore your flies can be slimmer and tinier, and more attractive for fish.
Tungsten nymphs are absolutely essential for fishing in deep pools and fast streams.