Because humans no longer die naturally as they did in the Age of Mortality, scythes kill people to manage the population. This act is known as "gleaning", after the early form of charity in which farmers would allow the poor to collect the leftovers from their harvests.

Why did scythe Faraday glean himself?

Scythe Faraday believed in using accidental death statistics from the Age of Mortality to choose his victims. For instance, he chose to glean a person because many people used to die saving their pets from drowning in rivers.

How does scythe Goddard glean?

He was gleaned by Scythe Rand in Kwajalein, after he attempted to stop the Thunderhead from sending colonists into space, Rand stabbed him in the heart.