Both Dreadlords and Pitlords are under the command of Kil'jaeden and Archimonde in the Burning Legion. However, some are more important than others, and some other demons seem to be special cases.
In the War of the Ancients,
Hakkar the Houndmaster was sent into Azeroth as a "lieutenant" - he, at the time, was a powerful demon that aided greatly in the Highborne's attempts to summon more demons. Later, they gathered enough power to summon Mannoroth, who was described as being much more powerful than Hakkar. Later yet, they summoned Archimonde, who was way more powerful than either of the other lieutenants. Dreadlords did come into Azeroth at the time as well, but none (that I can remember) were named, and they didn't have any sort of special status like Hakkar and Mannoroth.
However, it seems that since then, the Dreadlords have a much more prominent role in the Legion. Balnazzar is a powerful Dreadlord that corrupted an entire holy order, and then cheated death. Tichondrius is a major fight in Suramar, and he's the leader of the Dreadlords. They're shown as much more calculating and cunning than races like Pitlords, who are generally more physically strong. Dreadlords have powers over shadow and undeath, which makes them very dangerous to Velen. It should be noted that they're one of the original demons, and they once revered an Old God - that's actually how Sargeras found out about the Old Gods originally. Despite the fact that one of the priest artifacts is a literal claw of an Old God, an enlightened race of magically enhanced demons that can easily raise mortals as undead is quite dangerous.
But, even now, some of the newer demons are shown as being important to the Burning Legion.
Brood Queen Tyranna, an Aranasi, was in charge of Mar'dum - a position given to her personally by Sargeras. Nowadays, it seems more likely that it's based off an individual demon's contributions, rather than what the race can do altogether.