In the Sämisch Variation...
White moves this pawn forward
In order to create this pawn chain
Solidify his centre
And prevent Black from using the g4-square
He wants to castle Queenside (0-0-0)
And attack on the kingside by advancing these pawns
Black wants to castle Kingside
Undermine the centre with these pawn advances
Or move this pawn in order to challenge White’s Queenside