It was a reasonable guess at the time that standing any closer together than 2m (ROW) risked breathing in more than enough of what someone in front of your had just exhaled along with contaminated water droplets. Locally we settled on about 3m (10') apart (and totally ignored the UK government's silly no more than 3 people rule since you invariably met other couples or families sometimes more than one at a time).
It wasn't entirely without foundation. 100um droplets fall out of the air before they get more than a couple of metres away from you.
I can smell vapers from about 10m behind them so I always wondered just how effective it was as a strategy. However, I managed to avoid catching Covid for over 2 years by following those guidelines.
Back then they didn't know that Covid was so infective that it was effectively airborne and only air changes could ameliorate that.
The optimum distance was as far apart as possible which is why when things got seriously out of control we had lockdowns.
Japan probably made the best fist of a difficult situation. (still a surprising proportion wearing masks at indoor venues today)