For a couple of years, Apple has required apps that need access to the barometric pressure sensor to ask for permission. Counterintuitively, they have categorised this sensor under 'Motion & Fitness'. Understandably, you may think this is not necessary and reply 'no'.
However, AvTraffic uses this to calculate pressure altitude and altitude AMSL. This is important for showing relative altitudes to other aircraft.
A new FAQ explains why. You can read more about it here