I'm placing my piece in column 5 because it gives me three O's in a vertical row (positions 5,3 through 5,5). This creates a strong threat - if my opponent doesn't block by playing at position (5,2) on their next turn, I can win by completing a vertical connect four. This forces my opponent to play defensively rather than developing their own threats. Additionally, this move doesn't create any winning opportunities for my opponent.