Why does Caruana kick Aronian's bishop? Why does Caruana push 11.h3 against Aronian?

[fen ""]
[startflipped "0"]
[startply "20"]
[title "F. Caruana v. L. Aronian, FIDE Candidates 2018"]

1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 O-O 8.d3 d6 9.Bd2 Bg4 10.c3 d5 11.h3 (11.exd5 Nxd5 12.h3) Bh5 12.Qe2 Rb8 13.Bg5 dxe4 14.dxe4 h6 15.Bc1 Bg6 16.Nbd2 Nh5 17.Nf1 Bc5 18.g3 Kh7 19.Kg2 Qe7 20.Bc2 Rfd8 21.b4 Bb6 22.a4 Nf6 23.Nh4 Qe6 24.Bd3 Bh5 25.g4 Bxg4 26.hxg4 Nxg4 27.Nf5 Nxf2 28.Bc2 g6 29.N1e3 gxf5 30.exf5 Qf6 31.Qxf2 e4 32.Rh1 Rd6 33.Bxe4 Rg8+ 34.Kf1 Ne5 35.Qf4 c6 36.axb5 Rg5 37.bxa6 Qd8 38.f6+ Ng6 39.Rxh6+ 1-0


Caruana's 11.h3 does nothing I can see but to kick Aronian's bishop back to h5, seemingly a better square.

Yet Caruana wins the game, against Aronian, no less. So has Caruana's 11.h3 actually somehow helped?

LUFT?

Is Caruana's sole purpose to make Luft without loss of tempo?

But this seems doubtful. Why make Luft so soon? Aronian still has both bishops, after all; so, if the purpose is Luft, then how can Caruana already know that the h-pawn will have been the right pawn to push? (In the event, if Luft is what Caruana wants then he soon gets plenty of it. Aronian trades the bishop for all three of Caruana's kingside pawns!)

PAWN STORM?

Perhaps Caruana is dreaming of an eventual g4 and a pawn storm against Aronian's black king. But from this position? I see no sign of it. Do you?

I'm stumped.

MOTIVE BEHIND THE QUESTION

Though Caruana's game against Aronian does indeed interest me, my question has a broader purpose. Opponents sometimes play moves like Caruana's 11.h3 against me. I do not understand h3 (or h6) when they play it any more than when Caruana does.

I understand kicking a knight by h3. I also understand kicking a bishop by a3, insofar as the king is not near. But kicking a bishop by h3? If one does not mean to follow with g4, then why bother?

I would like to understand.

COMPUTER ANALYSIS

If you wish to know, Stockfish slightly prefers 11.exd5 Nxd5 12.h3, which is not wholly dissimilar to Caruana's play, so apparently Stockfish understands Caruana's thought. Unfortunately, I do not understand it. If you can illuminate the push of the rook pawn in a position of this kind, I would appreciate it.
You are right, h3 has no immediate purpose. But that's not the point. The idea behind playing h3 is to set up many resources that may be used in the future. I'll list these resources below:

A) Luft. Sometime long down the road, the king may need a luft to prevent a back rank mate. Yes, Caruana doesn't know if moving the h-pawn will be the ideal luft in every possible situation, but usually moving the h-pawn works best. You also mentioned that Caruana would soon have plenty of Luft because Aronian trades his bishop for the three kingside pawns. This is also true, but Caruana had no way of knowing this ahead of time.

B) Possibly playing g4. Notice how Black's knight on f6 is tied to the defence of the d5-pawn. One day, White may play g4-g5, booting the knight from its post and winning d5. This doesn't work in the current position (why else would Caruana play 12.Qe2 instead of 12.g4?), but again it's about the possible future. In some future position, Caruana may find the g4 idea useful. It's likely he will never use it, but it doesn't hurt to have the option ready should the need arise.

In summary of points A) and B), h3 is meant to set up resources that Caruana might use in the future. He may never use these resources, but it's good to have them ready just in case. Playing h3 doesn't hurt his position, so why not?

There's also two more reasons behind playing h3:

C) The bishop on h5 is arguably worse than on g4. On h5, the bishop can only go to g6, where it will be blocked in by White's e4-pawn (provided White can protect the pawn). On g4, the bishop would have been able to retreat along the c8-h3 diagonal.

D) As user1583209 pointed out, by playing h3 White has the option to suddenly break the pin with g4 at any moment. This may be a very useful resource, if White's knight needs to be freed up to do something immediately. Without inserting h3, Black would be able to capture on f3 if White played h3 later on.