The cartoon that Shaker put up recently answers that. To save us from what he will do if we don't believe in him!
And that is why the cartoon is so poor. If anything, its to save us from what we will do to ourselves if we don't believe in Him. OK, if you don't believe that humanity is a pretty nasty piece of work, happily destroying each other and the world in which it lives, then you don't see the need for humanity to be saved from itself. Those of us who see events unfolding on the news that illustrate man's inhumanity to man believe differently.
Regarding what God will do to us if we don't believe in Him - the Bible makes that clear: he'll leave us to the consequences of our own choices. In fact, he doesn't actually
need to
do anything. It is we who condemn ourselves to whatever our future holds.