I'm playing in column 5 for two strategic reasons. First, it creates a strong horizontal threat by connecting with my existing O pieces at positions (3,3) and (4,3), giving me three in a row. If I get another turn, I can play in column 6 to complete four in a row horizontally. Second, this move blocks a potential diagonal threat my opponent could create with their X pieces at positions (3,1) and (4,2). By playing in column 5, I'm both creating my own winning opportunity and preventing my opponent's.