I thought that was obvious: Stop Sam from going down *that *road. And showing the backstory served (within the mythos) to motivate Dean that even though Ol’ Yellow Eyes is dead, the threat is not over.
For us, on the other side of the fourth wall, it really connected the dots. I doubt this was only the table when they started plotting the show, but it’s a great credit to the showrunners that they actually make things fit within the arc/mythos without it feeling contrived.
Supernatural is not the best show ever, but it’s well contained, the writing is tight, my suspension of disbelief is seldom disturbed and the quality is steadily getting better. Also, I love that the show is a lot smarter than what it presents itself to be. Too many shows try out for smart and are actually dumb. Supernatural goes for ‘dumb’ but is really very smart.
An aside, since this is leaving the show and bordering on philosophy/religion. In a way, this reminds me of Joan of Arcadia (sadly missed) in that the Angel (or God in JoA) doesn’t really do anything. He only suggests certain paths. How does that fit with the canon (so to say) of Christianity, i.e. not only the Bible but also tradition and lore?