Getting an army all at once saves a lot of time and effort. You make a single purchase and boom, you're ready to go (after the glue dries, of course). That's the offer that Mantic is giving you right now, with a pair of Basilean army bundles for Kings of War. Get a whole army in a single go.
From the announcement:
If you're a devout follower of the Shining Ones, then we've got some good news for you. After what seems like forever, the Basilean Mega Army set is now available, alongside a brand new Basilean army bundle.