After 7.Bd2 Black has three options:
Black can pull the dark bishop back. Retreating to c5 doesn't work, since White can just push the b pawn forward, and force the dark bishop to retreat again, so you'd have to go back to d6 or further. The issue with this is tempo - you've giving white a free turn to develop.
You can try to use your Queen or the a pawn to defend the bishop, but these are a little awkward. In the pawn's case, White can exchange for the bishop anyway, and you are left with doubled pawns on b, and now you have to defend the forward pawn or just coincide it. The Queen defense line is more complex, and I'm not sure I'm experienced enough to do it justice. Bottom line is, your Queen is either tied down defending that bishop, since you can't get any other support there easily, or White is going to exchange for the bishop (again) and either push your Queen around or try to exchange for it.
Or exchange for the bishop. Your example ends with White's Queen taking the dark bishop. This doesn't really develop White's Queen, so you gain tempo by following it up with a development move - maybe the knight f6 you mention in your question.
Bottom line: the bishop is probably going to be exchanged, so you might as well do it in a way that doesn't give White anything. Moving the White Queen one square forward isn't much in the way of development, so that seems a good exchange.