Wikipedia points to a source that says it is used for parts of a multi-stage fusion bomb:
> Uranium with enrichments ranging from 40% to 80% U-235 has been used in large amounts in U.S. thermonuclear weapons as a yield-boosting jacketing material for the secondary fusion stage
Just to be clear, this isn't "useful [to make] a bomb" - it's useful in a thermonuclear warhead that already has a primary fission stage using the originally-mentioned highly enriched weapons grade uranium, plus a second fusion stage that (as far as I'm aware) Iran is not working to develop.
edit: phrasing. it feels like we're going around in circles nitpicking based on a poor framing and the tendency for innuendo on this topic
> plus a second fusion stage that (as far as I'm aware) Iran is not working to develop.
All modern nukes are two stage designs, Iran would be insane not to have a fusion stage. It would basically be a Hiroshima style dirty bomb with just 1.5% fissible mass actually fissioned.
It turns out there's a big gap between most peaceful purposes and weapons grade, and this was in that gap.