Hi! The idea is that the new users first test the printer without the MMU2S upgrade to verify it is assembled and works correctly. If the printer didn't work correctly before it was upgraded to MMU2S, the resulting issues might be harder to chase and might be assumed to be caused by the MMU2S unit itself. Also, this way there are parts necessary for the possible downgrade already installed.
For the new users, it is crucial to understand how the printer works as a single-material before they proceed to upgrade it to multi-material. This is why there is no guide to building the MK3S+MMU2S straight from the kit.