I need to block my opponent's diagonal threat. With X pieces at top-right (0,2) and center (1,1), they could win by placing at bottom-left (2,0) to complete the diagonal. By placing my O at (2,0), I prevent this immediate threat.

Additionally, this move creates a strategic advantage for me - I now have O pieces at top-left (0,0) and bottom-left (2,0), creating a potential vertical win opportunity if I can place at middle-left (1,0) in a future turn.