This is a topic that reoccurs every 18 months or so.
Just to paraphrase my usual response...
What is 'obvious' to one pilot may not be 'obvious' to the other.
A pilot can obviously have an incorrect view of the situation ((i.e. wrong mental model)). It doesn't mean we are substandard and need immediate suspension, it just means we are human.
One of the best ways to see if your mental model is wrong is to compare it to the other pilot.
We can only do that by communication.
And since we all know the flight path is the most crucial thing, it therefore follows that anytime anything about it changes we need to make sure our mental model is correct. So we need to announce what we think is happening, so that our colleagues can set us right / be set right.
CPB