Why do Agents Want Synopsis, Blurbs, etc

Why do agents sometimes want a synopsis and other times want the complete manuscript and a synopsis?

I’ve always had opinions about this but here is a post by an agent who explains why he asks for specific submissions packages.  What does he read for when he asks for a synopsis vs when he asks for the first chapter, the first three chapters or the entire manuscript each with a synopsis.  While Scott Eagan doesn’t represent children’s authors, this will give you some insight into how agents think.