Playing in column 1 creates a vertical threat with three O pieces in a row (at positions 1,3 - 1,4 - 1,5). This sets up a potential win if I can later play at position (1,2). My opponent may need to block this threat on their next turn rather than pursuing their own strategy. While my opponent could create a horizontal threat in the top row by playing in column 5, I can block that in my next move if needed.