I'm placing my piece in column 6 to extend my current two-piece sequence (at positions 4,5 and 5,5) to three in a row. This creates a potential winning threat and forces my opponent to respond defensively. While my opponent has three X's in a row, they cannot immediately connect four since position (4,5) already contains my piece, blocking their horizontal threat. Playing in column 6 is offensive rather than purely defensive and gives me more options for future moves.