why << >> in \score?

[Francois Planiol-Auger]

Hi,

Thanks to Maurizio, I nested the different parts with << {} // {} >> and it
works fine.

But I would be able to make better work if I could understand why making
expression for everything is for lilypond important? Expression for the
whole score, for singers parts etc. Why doesnt it work only with a syntax
like:

\score {
        \new StaffGroup {
                \new Staff {
                        \new Voice { ... }
                        ...
                }
                ...
                \new Lyrics {
                        \new Stanza { ... }
                        ...
                }
        }
}

Why must be used these << that makes for the beginner a big difficulty for
building a system? Why are the brackets not enough?
Why is it not necessary for Lyrics parts?

François, asking maybe a very silly question...