I don't really get the point of it either. If it's a video I want to watch, I would like to watch it from the start. That would require me to be there at the start of the livestream, like an appointment, which I don't think is worth it, just to live-chat with people.
Like, I'm sure there's some people who don't mind missing the start or enjoy the live-chatting a lot or just happen to want to watch a video at the time of the premiere anyways. For those, the concept works out, but otherwise it doesn't feel like it will ever really attract the masses.