I can give you both, multiple reasons
1. Currents (obv something Maries doesn't come into contact with)
Ascent alarms can give you the first indication of an upwelling that you might not "see" especially when you can't see the bottom
In a downcurrent washing machine - your spatial awareness can be temporarily disabled - ascent alarms give you an indication of travel
Coming off a reef (in current) - and needing to shoot your dsmb pronto. Ascent alarms can be of value
In proper current when it's all going a bit south you are somewhat task/bandwidth loaded staring at yoru computer might be low on the list of immediate priorities
For reference I probably engage in 10 or so significant down currents per year at least once a year it's heart thumping event.
2 Training and rescue
More than once I (as a Guide/instructor) have had to take control of divers on their ascent - my hands are full, so ascent alarm provides immediate feedback while I'm engaged in other more pressing issues
Similarly on Rescue courses, when I'm the victim - if my alarm goes off on the ascent, the student is briefed to stop and restart (on the deeper exercises)
So that's a couple of real world example without even thinking of it