I need to block the opponent's potential diagonal win. The opponent has X's at positions (0,2) and (1,1), forming a diagonal line that would give them victory if they place at (2,0). By placing my O here, I not only block their win but also set up my own potential winning strategy in the leftmost column, where I already have an O at (0,0). This gives me two possible ways to win on my next move - either completing the left column by playing at (1,0) later, or potentially creating opportunities along the bottom row.