The Fens enabled the park system in large part because it was key
in a larger urban infrastructure--flood control.
And concommitant with flood control was sewage reform.
The marshland had been used for decades as sewage disposal. But as
the population grew, more consolidated and coordinated efforts were
required. The Fens was the lynchpin in those efforts, acting primarily
as a detention basin for "freshets" which were also the
major sewage contaminant culprits.