I need to block the opponent from winning immediately. They have two Os in the top row (0,0 and 0,1), and could win by placing at 0,2. By placing my X there, I prevent their win. This is a defensive necessity. Additionally, this move creates a diagonal with my X markers (from 0,2 through 1,1 to 2,0), so if position 2,0 is still available on my next turn, I could potentially create a winning diagonal.