The main reason for this advice is if you have mirrors or glass that hands often touch. They need soap or detergent to remove the grime and then wipe it away with a microfiber cloth. Then proceed with a clean cloth and vinager to really make the glass shine.
Word of caution> Do not try to clean windows when the sun is shining directly through them. The glass will be to warm and the spray will evaporate before you have a chance to cleaning it off, and it will leave you with streaks.