Why aren't all wires used on the Honeywell T4 Pro thermostat?

Not all wires are required because different HVAC systems need different wiring configurations. The thermostat supports various system types including single transformer, dual transformer, heat-only, and heat/cool systems. Some terminals may not be needed depending on your specific setup, and the optional THP9045A1023 Wire Saver Module can reduce required wires by combining functions like fan and compressor control.