I don't know what engine you are using but Stockfish comes up with completely different moves.
A agree 1.b4 is good because the knight will find a nice square on c5. For example 1.b4 e5 2.Nb3 exd4 3.Nc5 attacks the Bishop. 3...Bc8 4.Nxd4 and white is better.
b4 would also stop b4 from Black which would attack the pawn defending d4. If the c3 pawn falls then you could be stuck defending d4 for the rest of the game.
There aren't many good moves in this position. My candidate moves are Rook e1 and pawn b4. Rook e1 to put the rook on a more active position and pawn b4 to strengthen your structure. It restricts the play of the dark squared bishop on the queen side however. A plan for this situation could be attacking the e6 pawn as it is the main weakness on the dark side camp. Cheers
If black plays ...b4 it significantly weakens whites d pawn.
That is what you are missing.
On the one hand white might be able to somehow save his d4 pawn during the next hundred moves but if he does he will need to protect it with his own pieces.
That would mean those pieces are now occupied with protecting duty and not able to attack black.
If white does end up losing the d4 pawn it is also a disaster since that would be losing a centre pawn which is more valuable than a wing pawn.
