tolkiens legendarium - What was the purpose of Sauron's human sacrifices?

When Sauron was in Numenor, he corrupted the locals into worshipping Melkor, offering him human sacrifices. The victims were mostly the Faithful, who opposed Sauron and his puppet king.

So, why did Sauron need those sacrifices? Was it only to eliminate his enemies, or an act of pure sadistic cruelty? Or could those sacrifices somehow mystically benefit Morgoth beyond the Walls of the World, or Sauron himself?

Answer

On balance, he was probably just manipulating Ar-Pharazôn

One of my all-time favourite of Tolkien's essays, "Notes on Motives", discusses somewhat Sauron's purposes in setting up "false" religions; most relevant, I think, is this bit:

[Sauron's] cunning motive is probably best expressed thus: To wean one of the God-fearing from their allegiance it is best to propound another unseen object of allegiance and another hope of benefits; propound to him a Lord who will sanction what he desires and not forbid it.

^{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (i)}

It was useful for Sauron to present Morgoth as the kind of God who would let Ar-Pharazôn do something he wanted to do anyway; namely, to revenge himself upon his ideological enemies (and recall that the Faithful had been ideologically opposed to the Kings for quite a while at this point, with only a short break during the reign of Ar-Pharazôn's predecessor, Tar-Palantir).

Sauron's ultimate purpose is also revealed in this essay: he wants to subjugate and dominate the Númenóreans, and bend them to his will:

Sauron was not a 'sincere' atheist, but he preached atheism, because it weakened resistance to himself (and he had ceased to fear God's action in Arda), as was seen in the case of Ar-Pharazon.

[...]

[T]hough Sauron's whole true motive was the destruction of the Numenoreans, this was a particular matter of revenge upon Ar-Pharazon, for humiliation. Sauron (unlike Morgoth) would have been content for the Numenoreans to exist, as his own subjects, and indeed he used a great many of them that he corrupted to his allegiance.

^{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (i)}