"Because there's a war coming, John. And if you're not back on the Island when that happens, the wrong side is going to win."
In The Life and Death of Jeremy Bentham, Charles Widmore visits John Locke and mentions a "war" coming. He tells John that if he is not back on the island when that happens, the wrong side is going to win.
Perhaps this war is simply between Jacob and Man in Black, and their followers of course. But if Man in Black is looking for followers on the island, it seems like he wouldn't find very many, seeing as all the others, Ilana's team, and [presumably] our Losties are against him, who exactly could Man in Black recruit? Perhaps "The Sickness", which we learnt more about in What Kate Does could be what will help him to get more followers. But even if he did get Claire or Sayid(which were revealed to have this "darkness" inside them) on his team, it seems like he would still be very much outnumbered. Which is why I think that Charles Widmore may also be on his team.
So Charles Widmore told Locke he had to be on the island when this war were to happen. Perhaps he wanted Locke to go back to the island [dead] only so Man in Black could take his form, and then manipulate Ben into killing Jacob. Jacob's death may have been necessary in order for Man in Black to win this "war", and then return "home", as Evilocke(Man in Black) revealed to Ben in LA X.
Perhaps Charles Widmore will play a big part in this war afterall.