Crescent's Edge was a campaign that benefited from having a shadowy character behind the scenes pulling strings. Who is the elf? What are his motives? Truthfully, at this point in the party's adventuring career, that doesn't really matter. The shadowy puppet master is two-fold: the adventures become connected, and the party feels that they are part of a much larger story.
Having an antagonist who is never present with the party is also useful. I've had recurring villains that always flee the party when they're overwhelmed, and the biggest complaint I've heard is that the party is robbed of the kill. Your typical adventurer doesn't really care if they get to see a villain more than once, they'd rather just kill it. With the puppet master, the party never has to physically meet the antagonist, so they don't have to feel robbed.
