Here's why:

* RNA viruses have RNA as their genetic material.

* RdRp is an enzyme that can synthesize new RNA molecules using an existing RNA molecule as a template.

* This is essential for RNA viruses because they need to replicate their RNA genome to create more virus particles.

Here's how it works:

1. The virus enters a host cell.

2. The viral RNA genome acts as a template for RdRp.

3. RdRp creates copies of the viral RNA genome, allowing the virus to multiply within the host cell.

Examples of RNA viruses:

* HIV (human immunodeficiency virus)

* Influenza virus

* Ebola virus

* SARS-CoV-2 (the virus that causes COVID-19)

Therefore, RdRp is a crucial enzyme for the replication and propagation of RNA viruses.