Linked from
The 6 pages that link to Altimeter, each with the reason it gives.
BarometerRelated: A barometric altimeter converts pressure changes into estimated elevation.
AutopilotRelated: Altitude feedback supports automatic altitude hold and altitude capture.
Flight instrumentsRelated: Static pressure lets it indicate altitude above a selected reference.
Wingsuit flyingRelated: Altitude awareness helps determine when to deploy the parachute.