Os dementadores se alimentam de desespero. O exército de Voldemort se espalha, permitindo que os dementadores se reproduzam. Os dementadores cuidam dos inimigos dos Comensais da Morte, levando suas almas e desmoralizando o resto da população. É mutuamente benéfico para ambos.
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
Então, vemos que estão causando desespero e outras coisas.
Nós também vemos que eles se reproduzem mais uma vez que Voldemort e seus seguidores se tornam poderosos:
"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
Além disso, (Kudos para @TheDarkLord por mencionar isso!) Voldemort lhes permite muito mais chances de usar seus poderes e se divertir:
"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