I'll add the small bit of information that I know.
My lama wanted a visiting lama to give us the empowerment for the 108 deities which live within our bodies. However, the visiting lama did not have enough time, so he gave us a Vajrasattva empowerment instead and said that Vajrasattva was the "summation" of these 108 deities.
OM BENZA SATO HUNG!
Yes, this is why many lama's will give a lung or pewang to students who have received a Vajrasattva empowerment rather than requiring them to receive the full empowerment before practicing specific yidams.
I never heard that in my 25 years as a practitioner. Which Lamas and traditions are you referring to?
In most Nyingma circles the base requirement for practicing ngondro etc. is a Vajrasattva empowerment. Usually if a practitioner has received the Vajrasattva empowerment and have access to their teacher they can get permission to do other practices through a pewang or lung rather than waiting for the entire empowerment.
I have experienced this many times with different Nyingma and Kagyu lamas. I'm not going to list specific teachers as I am not a fan of name-dropping. Sorry.
You very well may have never heard of or experienced this kind of transmission. Thats fine and I dont think you are incorrect.
You have your experience and I have mine, some people may share in what I have learned and others may have the same experience as you. In the diversity of teachings there is no one answer.