Each replication fork requires both leading and lagging strand synthesis. why?

Because each strand acts as a template for the synthesis of a new, complementary strand.

Explanation:

This is known as semiconservative DNA replication. Leading and lagging strands are complementary, antiparallel strands (one is 3'-5' while other is 5'-3- direction) which are replicated differently. The leading strand is replicated  continuously while the lagging strand is replicated in fragments-Okazaki fragments. Replication of both strands is performed by DNA polymerase.

when it is at an intermediate population density