I'm playing in column 5 to create a diagonal threat with my pieces at (3,3), (4,4), and now (5,5). This forms a diagonal line of three O's going up-left from the bottom-right. My opponent will be forced to block at position (2,2) on their next turn to prevent me from winning. This is the strongest strategic move available as it forces my opponent to play defensively rather than build their own threats.