I'll keep this review short to avoid any spoilers.
91 Days Special is mostly focused on the Vanetti family and how they rose into power, while the later part of the episode is focused on the interaction between Angelo Lagusa and Nero Vanetti. All these information contributes to the overall story of 91 Days; givings us an idea behind the characters' motivations.
So, should you watch this episode?
I'd definitely recommend watching it since it add details to the overall story. But of course, you can still enjoy the main series even without watching this so the decision is up to you.
