Hi Elouan.
You're asking a very good question.
The #1 reason is because by having a visual bone that indicates that "Hey this is the angle" is the easiest way for it to make sense to the most amount of brains.
ie - If you can see it, it's easier to understand what's happening.
But there are many different ways you could fire this when it needs to fire.
In a real rig you would never animate the DEF bone, so you need to make sure your solution doesn't rely on the wrong bone inputs, and that it only fires under the right conditions.