>>103067881To them, 3D debut is more of a very specific, special kind of stream/video, just like how some vtubers consider themselves pre-debut. It's meant to be a celebratory milestone or something. Usually they try to make the 3D debut something personalized and is expected to represent them long into the future.
Personally, my favorite examples of 3D debuts are Fuwamoco, Gavis Bettel, and Shiori's. Each one was extremely important and well thought out in portraying exactly what kind of streamer they wanted to be known as.
(As much as this place hates the homos, I highly recommend checking his out. Very 00's youtube humor) 