I'm playing in column 1 as a defensive move. My opponent has three Xs in a row horizontally in row 3 (columns 2-4). If I don't block column 1, they could play there on their next move (piece would fall to row 4), and then play there again later to create a winning 4-in-a-row. By placing my piece in column 1 now, it will fall to row 4, preventing this threat. While this move doesn't create an immediate winning opportunity for me, it's crucial to block my opponent's potential winning sequence.