When people say that Castiel isn’t important to Supernatural because he wasn’t on the show from day 1, I like to remember that many TV shows have been greatly improved by cast additions or changes, even though the fans grumbled at first.
MASH is probably my favorite example. People thought the show would die after the loss of Henry Blake and Trapper. But Col. Potter and BJ stepped into the roles and brought so much heart that they became fan favorites and the show was actually much better with their dynamic. Not to mention the swap of Burns to Winchester. Where MAJ Burns was a caricature and pretty much a joke, MAJ Winchester came in and was a fully realized, dynamic, layered character who again made the show all that much better.
How about Woody from Cheers? Who would have ever thought someone could have brought as much heart to the show as Coach? But Woody was perfect, not to mention Rebecca and Frasier, another character so good he got his own show that went on to be #1 for years.
The Walking Dead added Hershel, who became the heart of the show for a few seasons.
Putty and Norm added great comic chops to Seinfeld.
Toph was a great addition to the team that saved the world in Avatar, the Last Airbender. And like Castiel, she stuck around long after Aang knew how to earth bend and it’s good that she did as she definitely improved the show.
Adding more than one female definitely helped Big Bang Theory. It is so much better with the added dynamics of Amy and Bernadette.
I’m sure there are more examples, but those are off the top of my head.
Supernatural is, at it’s core, the story of two brothers. However, their interactions with others is what makes the program interesting. And, to me, the addition of Castiel has saved the show from endless MOTW repetition. He provides extra heart, endless story lines, that special perspective of someone who started with a completely different world view and was so influenced by the Winchesters that his whole life (and that’s saying something for a being around since the big bang) was changed and his character’s growth is just astounding.
Sometimes a side character resonates with the audience in a very special way, and occasionally even more so than the main characters. For me and many others in this fandom, Castiel is one of those cases. And that is why I think SPN is so much better with him in it. I could personally, watch endless stories of his personal growth and understanding of humanity, his internal struggle to become more than just a tool of the garrison, as well as his deepening complex relationship with the Winchesters.