Here's why:

* DNA polymerase, the enzyme responsible for building the new DNA strand, can only add nucleotides to an existing strand. It cannot start a new chain from scratch.

* Primers are short sequences of RNA nucleotides that provide the necessary starting point for DNA polymerase. These primers are synthesized by an enzyme called primase.

* Once the primer is in place, DNA polymerase can bind to it and begin adding complementary DNA nucleotides to the template strand.

So, in summary, the synthesis of a new strand of DNA begins with a primer, which is a short RNA sequence that provides a starting point for DNA polymerase to add new nucleotides.