That's a good point!
But I will mention this just as something only a dummy like me would do. Not that this actually happened, of course.
Say that this person didn't drain the water out, and the chiller was fine. But then, they stuck it in the brewpot to sanitize (an immersion chiller) before hooking up the hoses. And then just suppose that they had no awareness of science and how the wort would cause the liquid IN the chiller to boil as well and so this water would expand and shoot boiling water out at the brewer standing right there.
Now I'm not saying this was me, of course. Because that would be admitting doing something very very stupid.
But after that, a brewer might find they should either a) empty the chiller by just holding it upside down or blowing through it, or b) hook the hoses up before placing the chiller in the wort to sanitize, or c) both.
I'm not saying this was me!