Originally Posted by scottmushroom
It doesn't. Unless you are a studio head that wants 3 moneymakers to be released instead of 2.
While I'm sure that plays a big part, I also think the fact that this will likely be the last film set in Middle Earth plays a larger one. No one has the rights to film anything outside of what is contained in The Lord of the Rings or The Hobbit, and it's very unlikely that they ever will.
While the story contained in The Hobbit definitely isn't enough to carry three films, the added material from the appendices of the LotR trilogy and the roughly 100 pages of notes by J.R.R. Tolkein regarding a re-write of The Hobbit (he planned to re-write the story to match the tone of the trilogy better) should provide enough material to do so.
I, for one, have no problem with it. Loved all three of the LotR movies, so hopefully I'll feel the same about these new films. Like I said, a part of it is surely motivated by money, but I think the larger part of it is the passion for the project shared by the creative team involved with bringing these stories to life.