They aren't just sitting on their asses doing nothing while waiting for a debut. They hang around, talk to management, test things out (even if everything goes to shit the moment they start a real stream anyway), think about future plans, often prepare debut song and assets and other things of that matter, build up relationships, it's a time well spent to prepare for a proper debut as a different persona. There is literally only one reason why it works for vshojo - they are hiring orbiters and already established streamers with connections, meaning this steps could be skipped. Neither Holo nor Niji has such a thing as orbiters, people who are not part of the company yet effectively are.