Tudo explicado no Príncipe Mestiço (ênfase minha).
“Why did you stay there all that time, Snape? Still spying on Dumbledore for a master you believed dead?”
“Hardly,” said Snape, “although the Dark Lord is pleased that I never deserted my post: I had sixteen years of information on Dumbledore to give him when he returned, a rather more useful welcome-back present than endless reminiscences of how unpleasant Azkaban is…”
[...]
“But you didn’t return when he came back, you didn’t fly back to him at once when you felt the Dark Mark burn—”
“Correct. I returned two hours later. I returned on Dumbledore’s orders.”
“On Dumbledore’s—?” she began, in tones of outrage.
“Think!” said Snape, impatient again. “Think! By waiting two hours, just two hours, I ensured that I could remain at Hogwarts as a spy! By allowing Dumbledore to think that I was only returning to the Dark Lord’s side because I was ordered to, I have been able to pass information on Dumbledore and the Order of the Phoenix ever since! Consider, Bellatrix: The Dark Mark had been growing stronger for months. I knew he must be about to return, all the Death Eaters knew! I had plenty of time to think about what I wanted to do, to plan my next move, to escape like Karkaroff, didn’t I?
“The Dark Lord’s initial displeasure at my lateness vanished entirely, I assure you, when I explained that I remained faithful, although Dumbledore thought I was his man. Yes, the Dark Lord thought that I had left him forever, but he was wrong.”
Harry Potter and the Half-Blood Prince - Chapter Two, Spinner's End
Então, basicamente, uma combinação de fatores:
- Snape explicou a Voldemort que ele sempre foi fiel a ele
- Snape tinha 16 anos de informações sobre Dumbledore para dar a Voldemort
- Snape também tinha informações sobre a Ordem para dar a Voldemort
- Snape ainda pode permanecer como Hogwarts como um espião (o que também significa mais mais informação), devido ao fato de que ele não aparata para Voldemort quando o Marca Negra queimada
Por que Voldemort ainda confia em Snape se ele não conseguiu matar Harry?
“If I had murdered Harry Potter, the Dark Lord could not have used his blood to regenerate, making him invincible—”
[...]
“Indeed, many of the Dark Lords old followers thought Potter might be a standard around which we could all rally once more. I was curious, I admit it, and not at all inclined to murder him the moment he set foot in the castle.