The Raven King's sudden involvement changes the entire point of the novel.
As I already proved in detail while answering another question, the man who appears in Chapter 67 to revive Vinculus is John Uskglass, the Raven King, who's been so much talked about and yet does so little when he finally shows up. It's very significant that he doesn't seem to care about the summons from Norrell and Strange: he never appears to them, and instead just comes to do his own thing and then goes away again. Our perception of the story of English magic changes. It's not all about Norrell and Strange: in the big picture, they're just pawns in the Raven King's game.
Up until this point, it has seemed as though Norrell and Strange have been the ones pulling the strings of English magic. Their different approaches to the theory and practice of magic, and the growing strife between them (admittedly fuelled by the gruesome twosome Lascelles and Drawlight) has been the focus of the novel. Like Childermass, we the readers believe that they hold all the power and the destiny of English magic in their hands, and that their choices will determine its future.
Up until this point, too, the Raven King has been spoken of only as a distant, shadowy, semi-legendary figure. Their attitudes towards him make one of the biggest differences in philosophy between Strange and Norrell, but we the readers have been left unsure what to think of him. His existence in the past seems likely, his continuing power in the present day doubtful; he has apparently never manifested himself or his power over the course of the book.
NOW, suddenly, up he pops, seemingly out of nowhere. We might assume he appeared because he was summoned by Strange and Norrell, but he doesn't go to them, and instead goes about his own business. Casually he performs the feat of raising a man from the dead, which was so far beyond Norrell's abilities as to perpetrate the entire fiasco with Thistledown; equally casually, he rewrites the entire prophecy that inspired Strange's involvement in the world of magic. It's humbling to see how helpless Norrell and Strange are in comparison to him. This is the true power of English magic, and we'd never really glimpsed it before.
All of this is mostly implicit in the text of the novel. The only explicit pointer the author gives us towards this conclusion is Vinculus's claim:
"So?" said Childermass, stung. "That is not so very trifling, is it? Norrell is a clever man - and Strange another. They have their faults, as other men do, but their achievements are still remarkable. Make no mistake; I am John Uskglass's man. Or would be, if he were here. But you must admit that the restoration of English magic is their work, not his."
"Their work!" scoffed Vinculus. "Theirs? Do you still not understand? They are the spell John Uskglass is doing. That is all they have ever been. And he is doing it now!"
-- Jonathan Strange & Mr Norrell, Chapter 67 (emphasis mine)
Admittedly Vinculus is hardly a reliable narrator, but on matters of prophecy, he does tend to know what he's talking about. He's already spoken many truths which nobody recognised or appreciated at the time. His very body is attuned to the Raven King's intentions and prophecies; I'd say he's worth listening to at the very least.
It's the Raven King who is pulling the strings, and has been from the very beginning. It was he, through his prophecy and his agent Vinculus, who brought Strange and Norrell together in the first place. He showed up in England for his own purposes, to resuscitate Vinculus and rewrite the prophecy, not because of the summons.
This reveal is part of what makes the ending of the book so stunning and unexpected. Another part, of course, is the disappearance of the two main characters to another dimension, leaving others to carry on their work. In some ways it's an unsatisfying ending – we've identified with Strange and Norrell all this time, and wanted to see a resolution to their conflict and a glorious future for English magic (not to mention Strange's marriage!) – but there's no doubt it's an audacious move by the author, and one that leaves us (or at least me) almost wanting to read the whole book again from the beginning.
This answer is based on my answer to a similar question at Literature SE.