Option 1: Grassley/Graham genuinely believe Christopher Steele's a criminal, the FBI doesn't realize it, hasn't looked into him much, and needs prompting Option 2: They plan to claim testimony from Steele and Fusion GPS can't be made public because they're under FBI investigation
The FBI is not compelled to act. But federal agencies take Senators' requests seriously. As they should. Grassley chairs the Judiciary committee, while Graham sits on the Judiciary and Appropriations committees. Those oversee the Bureau and its budget.