When to play h3 followed by g4? (or h6 followed by g5)

Hi guys! I have played chess for about 8 months now, and I can understand mos chess ideas, but I never know when to play h3 challenging the bishop on g4 to move to say h5 and whether or not to play g4. Ofc one can also try this with black, here's an example. I do not play this opening myself but this was just meant as a simple example. I've seen this idea in the caro-kann as well. I know a lot of beginners who love this idea, but I've seen gm's also play this. So my question is when should I go for this kind of sequence and when should I refrain from doing so?  

Negative points:

Like all pawn moves, the pawns themselves are generally weaker the further they advance.  All pawn moves also permanently weaken the squares they're no longer able to protect.  (For example the black pawn on g5 will never have the opportunity to influence f6 or h6).


Positive points:

Pawn moves gain space.  More space for you to coordinate your pieces in that area, and less space for the opponent.  Pawn moves can also displace enemy pieces as they advance (like a bishop on h4 ).  These pawn moves can also be aimed at a pawn break where lines are opened.

Although, the player who has more developed pieces / more active pieces in that area will be the one who benefits from opening lines... if this player is your opponent he may use your advanced pawns against you and take the opportunity to play a pawn break of his own.

So when do you play h3-g4?  When the pros outweigh the cons of course!  Is this always easy to judge in positions you're not familiar with?  No

In the diagrammed position one important point is black hasn't castled kingside.  Good or bad, black's kingside has definitely been weakened... but because his king isn't there, and because white isn't more mobilized in that area, this really doesn't matter much.

In the positive points category black will have confined white's dark bishop to a passive square (if white moves Bg3).  Other than having a general plan of moving his pieces towards the kingside, castling queenside, and finally opening the kingside, black may find himself in a position to seek play on a different side of the board (or in an endgame) where white's dark square bishop is temporarily or permanently entombed on g3.  In fact I think there are some classic games with positions like this.

Anyway, so what to do when facing this.  Maybe you're white and you're not so sure your opponent is a grandmaster and you want to try to punish this advance.  If you have more pieces developed, especially if you're castled queenside or he's castled kingside, then aim for a pawn break like f4 or h4 (in the diagram) to open the position.

Or maybe your pieces can make use of the squares left behind.  Imagine the position where black's light square bishop has been traded off earlier, in that case the square f5 is likely great outpost for a piece.  Other "holes" (squares that can never be defended by a pawn) make for good outposts or infiltration points include h5, h6, and f6.

And of course there are the immediate tactics too.  White can also sac a piece for two pawns in the diagram by playing Nxg5.  When playing g5 black has to make sure after Nxg5 hxg Bxg5 that the threat Nd5 doesn't win back a piece and leave white with a winning position.

As a final note, there is some old rule of thumb that says not to play Bg4 (or Bg5 as white) before the opponent has castled precisely because of the h + g pawn can advance with a gain of time against the bishop.

I have a perfect example from a game I recently played IRL:

Beautiful played Boogie, thanks for posting that one!

Nice going !