What is the use of external power supply (3.3V) in the HDMI test setup?

What is the use of external power supply (3.3V) in the HDMI test setup?

The below diagram illustrate the schematic for one TMDS data pair. To measure the Voltage across the data channel you need Rt and Avcc. The test fixture has 50ohm resistor as RT, the test fixture requires an external power supply 3.3V which is applied externally.