But what exactly does that mean in the narrative? Why bother having it?
It seems to me those questions are not strongly related.
In the gameplay, it is a rare event that increases swing in combat, which keeps combat resolution from becoming too predictable.
In the narrative, half the time it means little - the average critical hit is roughly at the maximum of a normal hit. So, half of critical hits aren't really special. The other half of critical hits attain things you can't get otherwise, and so narratively... they're lucky shots that do more damage.
It isn't all that deep, honestly.