To help Jacob or his people. Can't be sure that Jacob could forsee everything. He might have just known that having chosen them, they needed to be there. Funny how once Locke died, Ben was the one working to bring them back, though for his own reason
I think that Jacob could foresee lots of possibilities, especially since he wrote the note before they even went back to the island, and when he appeared to Hurley in ghost form, he told Hurley about the hole outside the wall around the temple -- the hole where Smokey had attacked the French team. Wonder how Jacob knew about that...?
I think the key here is the conversation between Jacob and MIB, regarding the cycle of events, where MIB says it always ends the same, and Jacob points out that anything that happens is progress. So I'm assuming the time loop is something they've been dealing with for a long while and the Losties are just the latest in a long line of loops. But, it would seem (at least from Jacob's actions) that the Losties are the key to ending it all somehow.