*SPOILERS*
Well then what's the nuke's purpose? It happened because it happened and that's how they all came back to the present? That's almost as strong as some people landed in 1977 while some others in 2007 just because. Oh wait.
The nuke was a byproduct of WHH. it happened because it had always happened. There's also some great thematic stuff going on with it: for example, by detonating the bomb, Juliet ends up causing the very pregnancy problems she was sent to the Island to fix. Jack, by going along with the plan, ends up creating the Hatch that would later crash his plane.
--
~Pinkie Pie is the best pony~
http://www.youtube.com/watch?v=SBpfwpOPXNc&NR=1&feature=fvwp