Castiel is a comically serious fallen angel sent to help Sam and Dean Winchester in their battle against Satan. He's an emotionless, hopelessly deadpan, secret badass who constantly fails to grasp the intricacies of the human condition. Basically, he's Spock in an overcoat.
Castiel arrived at the opening of Season 4, promising a much more grandiose mythology for the show to riff on. His wasn't your typical "monster of the week" storyline, which the show had pretty much driven into the ground after three seasons. The emergence of Cas hinted at an epic tale of good versus evil that would propel the show into its finest hour: the Season 5 finale.
Castiel may have been unfeeling, but his injection into the show ironically proved its capabilities as an emotional powerhouse. And it's inconceivable to think that the pre-Cas version of Supernatural -- find monster, kill monster, discover much more evil monster on the horizon, repeat -- could have lasted nearly 300 episodes.
Also, let's not play coy here. The dynamic between Dean and Castiel is much more satisfying than the angsty, brotherly love/hate thing Dean and Sam have been trotting out for over a decade. Their fictitious friendship is as strong as any that's ever been written for a television serial, and a major reason viewers keep tuning in.