The move Bg4 in the Pirc defense I am having trouble understanding a move studied in an opening book. The move is Bg4 in the Pirc defence in this position.


What is the point of aiming to exchange the bishop on f3? The point d4 does not seem to be the reason.
Compare that position with other variations where White does not allow ...Bg4 to be played. Black often struggles with the development of the c8 bishop.

Also, the central dark squares are indeed a target, so ...Bg4 is clearly helping there (what else could he possibly do in a different square?) Pawn breaks with ...e5 or ...c5 are a possibility as well