De spredde både död, mörkhet, förstörelse, skräck och förtvivlan. Death Eaters gör det möjligt för demenserna att ta människors själar - ministeriet gör det inte. De är båda ... onda ;)
Dementorerna matar av förtvivlan. Voldemorts armé sprider den, så att demderna kan uppfödas. Dementorerna tar hand om dödens äterfiender, tar sina själar och demoraliserar resten av befolkningen. Det är ömsesidigt fördelaktigt för dem båda.
The Prime Minister sank, weak-kneed, into the nearest chair. The idea of invisible creatures swooping through the towns and countryside, spreading despair and hopelessness in his voters, made him feel quite faint.
Harry Potter and the Half-Blood Prince, chapter 1
Så vi ser att de orsakar förtvivlan och saker.
Vi ser också att de odlar mer när Voldemort och hans anhängare växer kraftigt:
"And if that wasn't enough," said Fudge, barely listening to the Prime Minister, "we've got dementors swarming all over the place, attacking people left, right, and center..."
Once upon a happier time this sentence would have been unintelligible to the Prime Minister, but he was wiser now.
"I thought dementors guard the prisoners in Azkaban," he said cautiously.
"They did," said Fudge wearily. "But not anymore. They've deserted the prison and joined He-Who-Must-Not-Be-Named. I won't pretend that wasn't a blow."
"But," said the Prime Minister, with a sense of dawning horror, "didn't you tell me they're the creatures that drain hope and happiness out of people?"
"That's right. And they're breeding. That's what's causing all this mist."
Harry Potter and the Half-Blood Prince, chapter 1
Också (Kudos till @TheDarkLord för att nämna detta!) Voldemort ger dem många fler chanser att använda sina krafter och njuta av sig själva:
"The rest of us sleep less soundly in our beds, Cornelius, knowing that you have put Lord Voldemort's most dangerous supporters in the care of creatures who will join him the instant he asks them!" said Dumbledore. "They will not remain loyal to you, Fudge! Voldemort can offer them much more scope for their powers and their pleasures than you can! With the dementors behind him, and his old supporters returned to him, you will be hard-pressed to stop him regaining the sort of power he had thirteen years ago!"
Harry Potter and the Goblet of Fire, chapter 36