You mean like Jim Kirk (both in TOS and the Kelvinverse)? Most of the Kelvinverse Enterprise crew? Saavik and the Enterprise cadet crew in TWoK? Wesley Crusher? Nog? The cadet crew on the USS Valiant? What's to explain?
No I don't. The narrative gave us an explanation for the Kelvinverse crew, and why Jim Kirk was on the the ship. From what I recall Saavik was brought on the ship by Spock. Wesley Crusher was on the ship because it was a starship that carried families and his mother worked on the ship. I can't speak about the other ones I don't recall watching those episodes/films.
Maybe I'm not being clear I'm not talking about fan inferred theories, I'm talking about the show giving an explanation in the narrative for why Cadet Sylvia Tilly is on a front line war ship in the middle of the war.
If a cadet is so important what not make her a fully fledged member fo the crew? What is gained by making Tilly a cadet? But then it's like a lot of the shows narrative choices that I end up questioning. What is the point?