I'm playing in column 5 to block my opponent's potential horizontal win. They currently have X pieces at positions (3,4) and (4,4). If they played in column 5 next, they would have three in a row, which could lead to four in a row if they later played in column 6. Additionally, playing in column 5 gives me two O pieces vertically (at rows 4 and 5), which could develop into a winning threat if I can continue building in this column.