The Hobbit, or, There and Back Again
Quick background: a Necromancer is a magician who works with dead things or who tries to summon the dead. We never actually see or hear anything specific about the Necromancer included in The Hobbit. We just know that he's the reason Gandalf has to go away south and leave the dwarves and Bilbo alone in Mirkwood. In a letter, Tolkien wrote that the original purpose of the Necromancer was "hardly more than to provide a reason for Gandalf going away and leaving Bilbo and the dwarves to fend for themselves, which was necessary for the tale" (source: J.R.R. Tolkien. The Letters of J.R.R. Tolkien. Edited by Christopher Tolkien and Humphrey Carpenter. New York: Houghton Mifflin, 1981, pg. 346).
So the Necromancer is just a plot device in The Hobbit. However, in The Lord of the Rings, the Necromancer gets a much bigger role – as Sauron, the Big Bad. And this place away in the south that Gandalf has to visit in The Hobbit must be Sauron's southern stronghold, the land of Mordor. So The Hobbit foreshadows not only what The Lord of the Rings will be about – Bilbo's ring of invisibility – but also who the main villain will be. The Necromancer may not be too important in the scheme of The Hobbit, but in Tolkien's larger mythology, he's pretty much the devil.