A traditional screen-sharing is: you have a screen, you allow other people to see it but only you can make changes i.e. there is only one mouse cursor and its yours.
ScreenHero was collaborative i.e. you were also able to allow the other person to control the screen i.e. each of you had independent mouse cursor/keyboard and could make changes at the same time.
I don't know of ScreenHero is the only program that could do that, but it's certainly rare.
It's also why implementing this is hard. Neither Windows nor Mac have notion of two cursor operating on windows at the same time, so I'm sure they had a lot of fun figuring out how to handle focus between two users.
I believe Slack bought ScreenHero team to implement screen-sharing functionality, although I don't think Slack's screen-sharing works the same way as ScreenHero. Haven't used it though, so might be wrong.
Either way, I think ScreenHero was differentiated enough to become very successful and now that it doesn't exist, there's a clear opportunity in essentially writing a clone.