They're seen across New York City, erupting out of the ground inside human encampments. Their purpose seems to be to disperse the Manhattan Virus in concentrated, instantly-fatal doses. They appear in Power Out and Dead Man Walking.
Two more massive Spears, the size of skyscrapers, are also seen in Dark Heart and A Walk in the Park. In both cases, Alcatraz must destroy the three spore conduits to increase spore pressure inside the Spears, which activates a safety system that allows him to enter the structure.
In Legion, it is speculated that the Spear in Central Park was intended to disperse a mass-produce version of the perfected, lethal dose of the spore throughout the entire USA. Further investigates were made by many scientists at the end of the novel, as Alcatraz stated that the Spore itself is even far more terrifying than it was initially thought to be, as Spore's would not stop at just the USA but also cover the entirety of Earth, destroying all of human civilization in process.