Answer: The block diagram of the Input section of a GPIO cell is shown below.
The input path has a Schmitt trigger that interfaces the physical GPIO pin to the internal data bus. In HighZ mode, this Schmitt trigger is enabled, thus connecting the pin to the internal data bus. So, for using a pin as a digital input, HighZ mode should be used. If HighZ Analog mode is selected, the internal data bus will always read a logic LOW irrespective of the voltage level on the GPIO pin.
In HighZ Analog mode, this Schmitt Trigger is disabled. This reduces oscillations on the Schmitt Trigger output when the analog input is at the Schmitt Trigger threshold. So, to use a pin as analog input or output, select HighZ Analog drive mode.