I noticed with the Focus, and other cars too, that they have an outside temperature gauge which can be made to display its reading inside the cabin.
Initially I had thought this was for the creature comforts, however I am now convinced that this is part of the 'mass air flow measurement' system. There was some disputes in this forum about whether it was a MAF or a 'XYZ?'.
Proper measurement of the air mass is critical to perfect combustion. Air mass can only be measured (correlated) by a Volume and a Temperature measurement. Fine adjustments are the role of the second oxygen sensor.