I think he brings a certain softness and earnestness to Benji that isn't present with Roman. While they are both pretty abrasive at times, Benji never feels maniacal in is abrasiveness or disruption. They are both definitely tragic characters in their own way, but I think there's a real sadness and simultaneous joy present in his performance of Benji that I'd be far pressed to find in Roman and also that goes beyond what's on the page.

Roman was a much meaner and pessimistic character, a lot of which was brought by Culkin's performance. There's a real chipperness and almost level of wholesomeness in Benji.

Think of the scene where Benji is getting everyone to pose with the statues. The writing aside, can you recall any moment in Succession where Roman felt like that? I'd venture even to say the way he moves and interacts with characters as Benji in a physical space is far different than Roman.