This ensures that Norton 360 (or whatever other antivirus you use for real-time protection) is registered with Windows as the main real-time antivirus. As bjm_ suggested, most Norton users who also have Malwarebytes Premium installed will turn OFF (disable) the Malwarebytes setting at Security | Windows Security Center | Always Register Malwarebytes in the Windows Security Center as shown below. I no longer use Norton as my primary antivirus, but I ran Norton Security and Malwarebytes Premium together in real-time protection mode for several years without any major issues.
See the Malwarebytes support article Issues Running Other Security Applications and Malwarebytes for Windows which includes a list of third-party antivirus programs that use the Windows Filtering Platform (WFP) that could conflict with Malwarebyte Premium's real-time Web Protection feature.