In most campaigns the DM has the general area and antagonists already outlined. "Kill some kobolds then move on to the temple with the undead." The enemy is often unnamed and ultimately of low consequence. Kobolds are plentiful and if for some reason the party fails to kill them the world will not end. Monsters tend to be generic, and if they have a name, no one in the world cares except for those living next door.
But as a campaign gets older the antagonists become more powerful to properly challenge the players. And it is at this point that some campaigns falter in the area of "believability". If the newest enemy is the OrcKing and his army of 100,000 Trolls, one of the first questions the players will ask is "why haven't we heard of this guy before?" "With an army that big where was he hiding for the past 2 years?" These sorts of questions will jar the players out of the immersion of play.
The more powerful an enemy is, the more world-wide recognition and impact they would have.
The trick is to feed the names of these powerful individuals and organizations to the players early in the campaign instead of having them leap out in surprise. This is easy to do if you know who the end-game enemy will be. You can slip their names out in passing..."The LichKing rules the North so no one ever goes there." "Thousands of trolls occupy MudFen so it's best to avoid that area."
However, sometimes a DM has not worked out all the details of his world yet. They may not know who the bad guy will be later in the campaign. That is okay. The trick here is to insert random names. Sure they will have no meaning now, but they will be available for later usage. Consider them placeholders. Examples are...the Crimson Divide, Cabal of Four, The Black Monk. You may not know what any of those are, but in a later adventure, at higher level, you can pull them out. The players will be already familiar with the name, even if they don't know the details.
One caveat to this article is that...sometimes it is okay to have an organization or powerful individual come at the players from out of nowhere, out of a vacuum. This especially works well for secretive type of groups such as secret police or hidden cults imbedded in the normal population. There the whole point is secrecy and for the players it makes sense to not have heard of them before.
But for big, world impacting power groups, get the word out early.