From what I understand, from school and people who have been around the business for a while, setting levels was/is very important for the analog broadcast signal. If the signal is too strong it can interfere with the neighboring frequencies. For a digital world there shouldn’t be any issues.
But the standards are still around for two main reasons. If every production company is required to set the output levels the same, the station engineer sets the fader to unity and doesn’t have to worry about the level strength being too loud or too quiet. On the viewer’s side, the views doesn’t have to continually adjust the volume every time a show/commercial comes on (however sometimes they do depending on the station).
Secondly, it prevents advertisement companies from competing on commercial loudness through limiting.
All my projects that head for broadcast have to be set a -20dBs, peaks at -10dBs.
I hope this helps
Kevin