It's considered okay before the 72 hours, so that all teams know when they can train generally, and when they can focus on tactical plays without fear of observation.
In that tactical window, teams can try out, and instruct on, a variety of options. Not just what you can see appearing in games.
Attacking and defending set pieces being obvious ones. In those, there are far fewer variables in a set piece than across wider play in the 90 mins.
We have it because Fair Play Bielsa was watching every training session.