I need to block X's immediate threat in the middle row. X currently has two marks at positions (1,1) and (1,2), and if they play at (1,0), they would complete a horizontal line and win. 

By playing at (1,0), I not only block their win but also start setting up a potential diagonal win from top-left (0,0) to bottom-right - I already have a mark at (0,0), and could potentially get (2,2) later. This creates more strategic options for me while preventing my opponent's immediate victory.