Hey @mr_kirby
Bg5 is just a nice developing move, but it isn't just about the tempo on the Queen. A less active developing move such as Be3 would also be okay. The reason the b3 move isn't as good is because pawn moves like these tend to leave many weak squares nearby. This is a more advanced positional concept, but after the b3 move, white has many weakened dark-squares such as a3, b2 and c3.
Weak squares should generally be avoided and this is one reason pawn moves require a bit of extra thought; they are more committal than moving pieces because pieces (Knights, Bishops, Rooks, Queen, King) can all move AND move back if needed, but pawns can only advance and can never move back. Weak squares are especially weak in front of a castled King and white's h3 and g4 in this position is very weakening for this very reason.
I made a YouTube video on this topic a year ago:
Hi all,
I am not that experienced with chess, and also somewhat new to the forums. But I had a question on a recommendation from the computer that came up when I was reviewing the game.
In the game, I played the pawn at b3 here:
I didn't really know where to play, but it seemed that advancing the pawn would develop my pieces more.
The computer wanted me to play the bishop here:
The idea was to get tempo against the queen.
I like the idea, but in the game, I was afraid of black playing the pawn at F6 to threaten my bishop (the computer analysis was going to play the black knight in front of the king).
If the pawn is played at F6, don't I just have to retreat with the bishop anyway? How is this gaining tempo?