Why is a 10dB attenuator installed between the measuring instrument and the artificial mains network during conducted emission measurement?

Why is a 10dB attenuator installed between the measuring instrument and the artificial mains network during conducted emission measurement?

This is specified in Section 4.2, AMN impedance, of CISPR 16-1-2.
The standard specifies the VSWR as seen from the receiver. (Specified value: 1 to 1.2)
To achieve this, a 10dB attenuator must be used either inside or outside the artificial mains network.