It’s more of a convenience feature, but important non the less. At times I have woken up before the wake-up-alarm and started with my day. Typically I forget to cancel the alarm manually and also leave the phone by the bedside, which in turn wakes or at least disturbs my partner’s sleep.
Automagically canceling the alarm when gotten out of bed before the alarm seems like a logical thing that should come out of the box 
Well, it gives me a feeling of security: that alarm is going to fire no matter what. But yes, it’s weird when it goes off in public in an early meeting/appointment. Still, I like the rock solid reliability. It’s a difficult choice, but I would go with letting it fire at the set time even if you got up way earlier.
Thanks for your input, @Robin_Markowitz!
Indeed, it is better to have the alarm go off. I made workaround to my issue by changing the ringing alarm to buzzing one.
Edit: now how do I close this topic?