Why we use 704
704 is divisible by 32 which makes it a rounder number in computing terms.
And since CCTV cameras generally output blackness in the missing "pixels" (although before sampling they're not technically pixels) no info is lost. 704 is not less resolution, it's less width - and that width difference, as well being black, generally falls off the sides of analogue monitors.
But what about viewing on a PC? Well if you make equipment that records "all" 720 pixels then your customers complain about the black ones down the side of the image!