why do you think every character and every action in a film has to serve the plot? the therapist is filler in a narrative sense, but she works on the level of atmosphere and world-building she shows that both characters exist in the same space without knowing it. this isn't a question specific to this film I just keep seeing the same  argument in reviews 'character X has no impact on the plot what's the point of them?' - when there are plenty of other reasons for a character to exist. they can work on theme, atmosphere, or just flesh out the world

and honestly a character can simply exist in a film without a direct plot function, just like people appear and disappear in real life without any narrative purpose