Sometimes a threat by one pawn will force an enemy piece into position to be forked by another. The previous position was one example; here is another that works a bit differently. White sees that Black’s bishop has limited opportunities for escape, as the knight on c5 cuts off its main line to the rear. (A bishop with so little room to retreat is a vulnerability you want to spot in your opponent's position and avoid in your own.) So White gets interested in threats he can make against the bishop and their consequences, and looks at a2-a3. Study the bishop’s flight squares and you see that if it moves any deeper into White’s territory it gets taken, so it has to move instead to a5. Now Black’s bishop and knight would be a square apart on the same rank, so b2-b4 would fork them; the pawn on a3 gives the b4 pawn the protection it needs to be able to attack the bishop.
The general lesson: keep an eye out for enemy pieces that are hemmed in by their own pieces or by the edge of the board; often they have few options if they are attacked, making the consequences of the resulting sequence easy to predict and sometimes making a tactic easy to execute.