I'm playing in column 5 because it strengthens my diagonal threat. I already have O pieces at (3,3) and (4,4), so placing a piece at (5,5) creates three in a row diagonally. This gives me a strong position and forces my opponent to block this diagonal or risk me creating a connect four in a future move. Additionally, this move starts building across the bottom row with my existing piece at (4,5) without giving my opponent any immediate advantage.