I know a lot of the early history, I worked there when we hired both Jack and Biz. I can tell you that it's a really good thing that Biz is back on board. Biz was able to articulate what twitter was as the human voice of the company in both directions. From day one the question was always, what does Biz do.
Biz bounced around but when he was there things were better. Kind of like the basketball player that Nate Silver likes to love, who doesn't have any stat which makes them star, but everybody else around them plays better when they're there. Biz isn't a business guy, nor product, nor code, nor support, nor really marketing. But when he's in the room, working with people, everybody's better at all of those things.
He can play a kind of court jester role, which is disarming, but he's super damned sharp. He uses stories and humor to bring people forward.
Having him there, working on twitter means there are now two people in senior roles who aren't afraid of breaking twitter, because they created it in the first place.
In recent years, talking to twitter employees you get this amnesia over the company's culture and history. People don't know where things came from, they don't know the story of how the came to be. The myth's are complicated and messy. And eventually go so messy the company stopped telling the story of how twitter came to be where it is now all together.
With Biz back, he can take on that internal story telling, creating a hero's journey that the company can believe in. Because he's there, as an equal to Jack in understanding the origin, he can tear things down without fear of destroying somebody else's house of cards.