If you remove the battery, I'm sure that you will realize that your indoor temp/humidity and barometric pressure would stop working on the main display. Another clue is that if you look at the outside sensor array unit you'll notice that it has a corrugated section which is a radiation shield. This indicates that it has a temperature and humidity sensor, which is well for outdoor because that is where that unit goes obviously to measure wind, and rain. Therefore the small sensor unit has no need to be outside for any other purposes. Barometric pressure is most always an indoor located sensor because there is no need to expose it to outdoor weather as the pressure indoors is the same as indoors so that is yet a 3rd clue. The reason it isn't incorporated into the main display is to mitigate poor indoor temperature and humidity sensor performance due to LCD display electronics generated heat.