Playing in column 1 serves two purposes: it blocks a potential horizontal win for X in row 4 (if they were to play at columns 1 and 4 to connect with their existing pieces at columns 2-3), and it starts building my own potential winning path. Column 4 already has two of my pieces, but playing there now would set up column 5 for a potential vertical win for X in their next move. Column 1 is safer and provides better strategic positioning.