Ulhumbrus: Here is a part of Bronstein's comments, from his book "200 open games":
<This is one of my most successful attempts at the difficult art of positional play. Simagin could not fathom the meaning of h4, and his King fell victim to a pawn breakthrough.
The subtlety of 29 e6 comes out in the variation 29...axb3 30 e7 Re8 31 Rd8 bxa7 32 Rxe8+ Kh7 33 Rh8+ Kxh8 34 e7-s8/Q+ Kh7 35 Qe1 Nd5 36 Qa1 Nb4, where Black is a Queen down, but in exchange has a strong pawn on a2.
White is saved by his own pawn on c5, which prevents the Black Knight from getting pawn support: by the march K-f2-e2-d3-c3 the white King will tip the balance in his favour. There is also a simpler solution: 33 Rd8! a2-a1/Q+ 34 Kh2 Qe1 35 e7-e8/Q Qxh4+ 36 Kg1.
There are probably many other compliated variations hidden in the position after 24 f4. I would like to draw the reader's attention to the importance of the open file for the Rook, be it either the f file or the d file >
So what does White gain by playing the move 23 h4?
To begin with, after 25 e5 White has a double potential threat of Rd7 and Rxf4 eg 25...Rxd1 26 Rxd1 threatening 27 Rd7 or 25...Rd6-d8 26 Rxd8 Rxd8 27 Rxf4.
Now suppose that White attempts to start his pawn breakthrough by playing 23 f4 and 24 e5 at once, without the preparatory move 23 h4.
After 23 f4 exf4 24 e5 White threatens to attack the f7 pawn by 25 Rxd8 Rxd8 26 Rxf4 but the difference is that now Black has the move 24...g5 defending the f4 pawn and obstructing White's attack on the f file.