I need to block my opponent's potential win. They already have Os in positions (0,0) and (1,0), creating a vertical threat. If I don't place an X at (2,0), they could win on their next turn with a vertical line. Additionally, by placing my X at (2,0), I create two potential winning paths for myself - one diagonal from (0,2) through (1,1) to (2,0), and another potential horizontal path across the bottom row if I can later secure (2,1) and (2,2). This is both a defensive and offensive move.