Why is b3 the best move here?

Middlegame Planning:

1. Expand your position:
2. Gain more space.
3. Improve the position of your pieces.
4. Decide on what side of the board to play.
5. Queenside: a-c files.
6. Center: d-e files.
7. Kingside: f-h files.

            Compare, space, material, and weakness(es)

            Play where you have the advantage.

3. DO NOT HURRY. Regroup your pieces, and be patient.

The c4 pawn in the current position is restricting white's piece activity. With b3, perhaps the strongest piece in black's position is traded off, activating the queen. After the trade, apart from having a hug development advantage, white is now also threatening a4, which will completely tear apart the black queenside structure

I think b3 is a mistake, ( due to low depth analysis by chess.com)

After b3,  black can play b4 , threatening Kt on c3 and then play c3. ( Kt on c3 give free tempo for black to advance pawns).

@drmrboss - I agree, b3 isn't great, and will be met by b4 by black, where the c3 knight doesn't have a quick path to any good squares.  White is kind of in a spot, though, his piece mobility is poor, his center is under attack, and there doesn't seem to be a good way to stop b4 from being played.  For example, if a3 then a5 renews the threat.

Looking at the original position, Bd2 might be better, vacating c1 to make room for the rook on a1 to move to a semi-open file.  Chess.com's analysis may not be too shallow, but it may be too classical.  Black's development may be poor, but the fianchettoed bishops, semi-open d-file, and queenside space advantage mean black is stronger where it matters most - in the center.

easy, destroy black's pawn chain with b3.

after b4 Na5 c3 Nc5, white has a nice square for the knight, and his bishop can be menacing on c4.

1.e4 is a premature center expansion, as Black can hit it with moves like 1...c5 or 1...e6 followed by ...Ne7 and ...c5. Notice also that the e3/d4 pawn chain keeps the g7 bishop temporarily in idle state.

Black is a pawn up, and the price he had to pay is falling behind in development. White's natural plan is certainly enough breaking that Black pawn chain at the queenside, so the easy moves to consider are 1.a4 and 1.b3.

1.a4 b4 2.Nb1 will win the pawn back (c4), and certainly is a good move, although all that takes time to redeploy the knight and Black will be able to develop normally. In general, I find white's position a tad more easy to handle in similar structures, but on the other hand Black's position has nothing fundamentally wrong other than a potentially vulnerable pawn at a5, but this could become relevant after many moves.

So, 1.b3 tries the same in a more successful way, as 1...b4 2.Na4 gives the knight a great park square at c5. You can see above comments that 1.b3 is poor, and after 1.b3 b4 the knight does not have any good squares, which is just a typical case of positional blindness- a4 is in reality a huge square for a knight. A black pawn chain at b4/c3 would be a worry only if it obstructed some white piece, which is not the case here.

My quick guess is that after 1.b3 Black has to take 1...cxb3 2.Qxb3 Nf6, and manage a difficult, but defensible case.

sometimes what this app shows isnt the best move

what do you suggest then?

@SNUDOO - After b4 Na4 c3 Nc5 gains tempo on the b7 bishop, but black wins that tempo back after Nd7.  If the c5 knight doesn't move, the exchange on c5 trades a black knight that has moved twice for a white knight which has moved three times, and forces the queens off, reducing the liability of black's still-uncastled king.

I'm guessing that white needs to leave the knight on a4 for a while for the b3 line to work, and that the option of posting to c5 later is worth having.  The problem with allowing black to create the b4-c3 complex is that it creates a protected passed pawn.  White must be careful to avoid long simplifying sequences (and these pop up all the time when one side has a double fianchetto, as black does here), as this is not something you want to leave for the endgame.

White may be nominally better after b3, but he's not out of danger by any stretch.  I suspect there are a bunch of only moves along that line, some of which may be difficult to find.

You have to be more specific, else your evaluations are thin air.

Just a sample, very casual try to analyse:

I'm lazy to analyse 1.b3 and 1.a4 properly.

I do think that both will lead to an advantage for white, but 1.b3 is certainly the most consistent, and pressing.

@pfren - Obviously, you are a stronger player than I am.  I'll graciously stand corrected.

The undeveloped g8 knight is obviously a bigger problem than I foresaw.  If it weren't in the way, a O-O would resolve the Qa6 threat quite comfortably.  As it is, there's no good place to put it after 7. Qd3! (which I'll freely admit I overlooked).

Black needs two moves to get his king to safety, but unfortunately enough white also needs two moves to wipe all the Black pawns on the queenside.

Thanks all for the feedback

*cough* 

^^

Since White can't win that pawn back, they use this pawn closer to the center to exchange it for the flank pawn to gain piece activity.