I recently played a game that opened like this:
1. d4 c6 2. c4 d5 { D10 Slav Defense } 3. b3
[FEN ""]
My third move, 3. b3 is apparently so uncommon that Lichess doesn't have it in its top 12 most common moves.
Stockfish rates the position after 2 moves at +0.2 and after 3. b3 at +0.0, so it's theoretically not terrible. However, the top 6 responses for black (topped by 3..Nf6 and 3..3Bf5) all have winning percentages favouring black (39%/54% for the knight), so there's clearly something wrong with it.
I often feel intuitively that I want to make this move (and in similar positions), reasoning that I prefer to bring my pawns towards the centre. Pawns on c4 and d4 look good, although I imagine b2 can become vulnerable.
OTOH, I can see that this move doesn't help develop any pieces, and maybe white is now going to be a move behind black after something like 3..Nf6?
I'd love some explanation about why this move is so dispreferred.