Why is there a leading and lagging strand during DNA replication

The double stranded DNA is first unzipped by an enzyme called helicase. Helicase separates the two strands of DNA, and creates the replication fork. Essentialy two single stranded DNA chains are created, both facing different directions. On the leading strand, an RNA primer is created by RNA ploymerase and DNA polymerase III will continously build that strand, since it is building the DNA chain in the same direction as helicase unzips the DNA. On the lagging strand, the DNA plymerase moves the opposite direction as helicase, thus it can only copy a small length of DNA at one time. Because of the different directions the two enzymes moves on the lagging strand, the DNA chain is only synthetised in small fragments. Hence it is called the lagging strand.