Actually, no I'm not missing the point.
The spring had snapped and not worn out.
I don't know if it was a fault in the spring or the wheel had hit a pothole previously. Either way, it just snapped.
If the OP uses a different brand or spec of spring than definitely both sides should be replaced.
I replaced with an identical MB spring with the same colour markings.
I had the car for about two years after I replaced one spring and the other side was fine.
I am not saying that you shouldn't replace both sides but just that it isn't mandatory.
Replacing shock absorbers is different. If one side is worn out, they should be replaced in pairs.