Final answer:
An RNA primer is considered essential for DNA synthesis by DNA polymerase III because it initiates DNA strand elongation.
Step-by-step explanation:
An RNA primer is considered essential for DNA synthesis by DNA polymerase III because it initiates DNA strand elongation. DNA polymerases can only add new nucleotides to the free 3' hydroxyl group of an existing nucleic acid strand. Since no known DNA polymerase can start synthesizing new DNA strands from scratch, a primer is needed. RNA primase synthesizes a short RNA segment, called the primer, that is complementary to the template DNA, providing a free 3'-OH group to which DNA polymerase can add DNA nucleotides.