Binaries Matter Everywhere: from Precision Calibrations to Re-Ionization and Gravitational Waves

Binary stars are pivotal for the coming decade, enabling our understanding of star-formation, stellar evolution, stellar death, supernovae, black hole formation, cosmic re-ionization, and gravitational wave events. The field needs time-domain surveys in photometry, astrometry and spectroscopy. The first two exist, the last not (yet)!