RTSP describes how a client controls media streaming and is widely implemented by IP cameras, NVRs and mobile camera applications. It does not standardize every camera setting, user account or analytics event. That is why a camera can have a working RTSP URL even when ONVIF discovery is absent or incomplete.
RTSP gives the VMS access to the actual video stream; ONVIF can simplify discovery, profiles and compatible device functions. Using both when available reduces manual configuration without making the stream dependent on a proprietary application. See ONVIF vs RTSP and Connecting an IP Camera.