Our opponent could be setting up a diagonal win starting from (row5,1) X, (row4,2) X by aiming for a diagonal sequence: placing a piece in (row3,3) would help him complete a connect-four in subsequent moves by filling (row2,4). Dropping our piece in column 3 lets it fall to row3 and blocks that potential diagonal threat. It also maintains our formation without giving a free win opportunity to him.