I'm placing my piece in column 5 to build a horizontal connection with my existing piece in column 4. This sets up potential horizontal win opportunities if I can later place a piece in column 6. This move doesn't create any immediate threats, but it develops my position on the right side of the board without giving my opponent any advantage. My opponent has three X's vertically in column 3, but I can't block that threat since there's already an O piece at the top of that column.