I'm blocking the opponent's diagonal win threat (positions 0,2 → 1,1 → 2,0). While I could also block their horizontal threat at 1,2, playing at 2,0 is better because it not only blocks their diagonal but also gives me a potential winning path in the left column (0,0 → 1,0 → 2,0) if I can get position 1,0 later. Additionally, this move starts forming a potential diagonal win for me from 2,0 to 0,2. Playing defensively while setting up offensive opportunities is key in Tic-Tac-Toe.