There are two operations on flash memory: "erase" (set all bits to 0) and "write" (set some bits to 1). The former is expensive, the latter cheap. Note that "

There are two operations on flash memory: "erase" (set all bits to 0) and "write" (set some bits to 1). The former is expensive, the latter cheap. Note that "write" can't set bits to 0. Hence the benefit of keeping zeroed blocks around.