I think the main reason Iran wants the bomb is the same reason North Korea wanted the bomb and in fact all other countries wanted the bomb: National security.

Nuclear weapons are not offensive weapons, certainly not when you're small enough so that any such attack on your part would only result in you being completely annihilated in retaliation. Nuclear weapons are an insurance policy against others deciding that you're ripe for "regime change" and/or invasion.

From what we know Israel also has the bomb and for the exact same reason.