3,000 Reasons

Iran claims that it now has 3,000 centrifuges online to enrich nuclear materials. This is just a waypoint on its path to getting tens of thousands of centrifuges operable, and such a cascade would enable Iran to produce weapons grade uranium.

That's always been the fear in Iran's nuclear push. They claim that they're doing this for peaceful means, but all the signs point to Iran's intent to use the materials for their weapons programs.

Ahmadinejad's repeated statements calling for Israel's destruction point to their intent to obtain weapons that would enable that goal.

UPDATE:
The 3,000 centrifuges was not the ultimate goal sought by the Iranians, but rather an intermediate step before having 50,000 centrifuges or more operable. They are looking at a multiyear plan to built an infrastructure that can support the ongoing production of enriched nuclear materials.