Scoring it makes the surface crack predictably - like saw cutting a concrete slab.
I'll be honest, we do this so often I'm not real concerned about how the bread looks anymore, sometimes I score it, sometimes I don't.
It all tastes good whether you make it look nice or not.
You can make a really beautiful boule if you want. This one had a little more stretching of the surface to help it keep it's round shape and I scored the top.