ROHC is an optional feature in PDCP.
For down link data packet PDCP performs the ROHC operation first and process further for cyphering (if configured).
During Voice traffic (VoIP), the size of the packets flowing through PDCP are very small with relatively big IP header and less data (32 bytes of payload and 60/40 bytes of header for IPv6/IPv4 respectively). To achieve big gain in bandwidth, PDCP uses RoHC to reduce the IP header size significantly, which in turn helps in carrying more payload data.