I think of it as a link between the dreamlike aura of John’s verses and the straightforward wide awake bounce of Paul’s middle. To me the orchestra flourish is like someone arising from a dream. Very chaotic and disorienting. I don’t think the song would have worked as well with just any link. The orchestra let’s people move between two very different songs, to me anyway, with less of a feeling that they’ve just randomly pasted together two songs.
I admit I do love ADITL as a recorded and arranged piece. The heart is of course John’s lovely verses which are brilliant. But I think it did need something in addition.