I'm placing my piece in column 2 to block an immediate vertical threat. My opponent has three X pieces stacked vertically at positions (2,2), (2,3), and (2,4). If I don't block, they could place in column 2 and win with a vertical four-in-a-row. My piece will fall to position (2,1), preventing this win. While there's also a potential horizontal threat in row 3, it would take my opponent multiple moves to complete, so the vertical threat is more urgent.