Warlords are the best buff machines in existence in 4e. Three of them, at level 27, can reliably kill Tiamat before she gets to act. Even without that extreme, they take one of the fundamental basics of 4e rules design philosophy - that bonuses on attack rolls are strictly regulated to end up in a certain narrow range based on level - and tear it to shreds. A party with a warlord buffing their attack rolls is an order of magnitude more powerful than one without.