In chapter 5 how are citizens useful even after death

They recover the phosphorus from the burning corpses. 

"Phosphorus recovery," explained Henry telegraphically. "On their way up the chimney the gases go through four separate treatments. P2O5 used to go right out of circulation every time they cremated someone. Now they recover over ninety-eight per cent of it. More than a kilo and a half per adult corpse.