I need to play at column 5 to block my opponent from creating a potential connect four. If I don't block, they could play there and have three X pieces in a row at positions (3,4), (5,5), and (6,5), needing just one more at (4,5) to win. This is a defensive necessity. Additionally, playing at column 5 develops my position in the middle-right area of the board, which could be useful for future moves.