For Iran, it's essentially aimed to regain military balance against its regional neighbors.
Iran is surrounded by countries (Pakistan, India, China, Russia, Nato, Israel) that all possess nuclear weapons, long range ballistic missiles, and observation satellites, and Iran is currently defenseless against such weapons. Any country in Iran's position would be looking for a deterrence against potential attacks (the invasion of Iraq has showed them that the threat is very real), and when you're dealing with nuclear powers, the only deterrent is to acquire nuclear weapons yourself.
But Iran goes beyond the simplistic "axis of evil" concept that the US wants to sell today, first of all Persia is one of the world's oldest civilization with over 5000 years of history behind it, and despite centuries of regional domination, Iran has never showed in modern times any will to make territorial gains over its neighbors.
2 interesting points that shows the other side of "evil":
The first
set of human rights were proclaimed by Persian king Cyrus the Great in 539 BC (interestingly he freed enslaved jews and let them go back to Palestine).
After WWII, Iran was on its way to democracy, but the democratically elected Iranian governement was overthrown in 1953 by… a
CIA coup to reinstate a corrupted monarchy. The coup was pretty much ordered by western oil companies, sounds familiar? Which side was "evil" there?.
Many young westerners wonder why Middle-Eastern countries have so much 'distrust' (to put it lightly) for the West, well… They should read some history books on the colonial period and especially how colonial powers and oil companies supported corrupted regimes that oppressed people in order to maximize profit.