Well personally, I don't think Israel would wage a first strike nuclear attack against Iran, for essentially the same reasons that I don't think Iran would wage a first strike nuclear attack on Israel. So unless Israel were to do such a thing, I don't see much point in speculating about why they might do it. I do see a possibility that Israel might want Iran to think that they would wage a first strike nuclear attack on them. I suspect that if this is the case, there are quite a few reasons they might do that. The biggest reason I can think of would be the deterrent factor.