Up until that point, we had not seen the Night King in the battle anywhere. None of the viewers know where the king is. Bran used the birds to find out where the king is at-presumably to gauge how long he has until their showdown. They made it a point to not show the king until then-and I know at my house we were waiting for that ice dragon to start fighting....
In a way, he's using the birds as a jedi would use the force to know when danger is nearby. By keeping tabs on the king now, he can now watch him, to see if things are going according to the future plan.