The fianchettoed dark squared bishop that is intended to be traded off of defends key squares, but I see little point in this trade. Black's king would still be safe on g7 after the trade.

Well, the key is that this bishop defends the dark squares now the king has to do that. The h6  square is really weak so black just needs another piece to help the invasion forces. White will follow up quickly with 0-0-0 , f3 to stabilze the center .  the structure of f7-g6-h7 has created a natural hook on g6 for the rapid opening of the h-file with h2-h4-h5 if black tries to block it then g4 is also a follow up sac. Often white will sacrifice rook for a knight . and swing the other rook over. The attacks are fast and brutal

See Tarzan Attack under Barry Attack CP. 5. Starting out D pawn attacks. Richard palliser.

" Against the direct Tarzan Attack Black has a large number of options, and the fact that he has tried so many is testament to the surprise value which 5. QD2 still has. Objectively Black is fine after aboth 5....Ne4 and 5....0-0, but white players who quite like rather complex early middlegames and who aren't adverse to the occasional positional edge may stiil do fairly well in practice."

The trade against the fianchettoed Bishop is thematic from both sides, but it isn't always good.  While the way to exploit weakened dark squares often begins with trading the dark square Bishops, this process takes a good bit of time:  Be3, Qd2, Bh6, Bxg7 and in the end doesn't even guarantee the dark squares will be weak enough to help an attack.  In the meantime, the opponent also gets to move, and in the opening this can mean a lead in development.  If Black gets ahead in development, it will be difficult for White to find an opening advantage.

But the Qd2-Be3 deployment isn't a commitment to a trade.  It can be a perfectly good development on its own.  As with most general rules in chess, it depends on the specific position.

agree with Estragon. Theres no 100% fixed plan of attack that is successful. The tarzan attack, 150 attack , yugoslav attack, english attack are all names of plans based on this type of idea. The english attack was used against a  variety of positions in the siclian but the same sort of developmental plan. 

e4, f3, Bg5, Qe2 , Nc3, (orNd2), Qd2, 0-0-0 . its a great learning experience for both sides to see howthings work