There'd be a point in having a weapon as a deterrent if you told your enemies in advance that you were not prepared to use it? You'll have to explain your contorted logic to persuade anyone of that.
It shows how badly the Corbynites misjudge thinks. This was supposed to be a "gotcha" question in which the new PM was supposed to look stupid and hypocritical (if she said that she would not use the nuclear deterrent) or like a monster (by forcing her into a stuttering admission she said that she would).
Instead, by quietly and calmly stating that she would use it, she completely pwned him.
Please note that I personally am staunchly pro unilateral disarmament and anti-Trident replacement.