It's simply good practice.
I've had bound partners have muscle cramps, which pretty much demands a full on stop asap, and a safe word can offer that. I've had partners have a repressed trauma emerge while they are bound in a scene, and having a safe word is pretty key there to know to stop and give them proper attention. I've had partners have panic attacks while playing, and again, a safe word is pretty useful here.
Now cases such as the above are rare, and happen a very very small amount of the time. And it's easy to never encounter one, or think you never will, but it's better to be safe and have a way to handle it than not.
I've had partners that sound like they are literally dying the one million deaths as we play, but they are fine and enjoy it. But it's damned hard to tell their sound suite apart from true distress.
Myriads