An altimeter tells you how far above the ground you are. This is essential information so that you can open your parachute before you hit the ground.
"Audible altimeters" make a sound at a pre-programmed height(s), e.g. the height where you should pull your parachute, or for formation skydivers "break-off" height where you start moving away from the other skydivers to prevent mid-air collisions when you open your parachute.
(Note that "Skydiving" and "Parachuting" mean the same thing).