This may simply be smarts on the part of the designers of the chip on the motherboard - either a motherboard vendor can choose to use it without external silico

This may simply be smarts on the part of the designers of the chip on the motherboard - either a motherboard vendor can choose to use it without external silicon andoughly get 1 USB-C channel or add the external switch and get 2 channels - it uses roughly the same number of pins for the two sorts of interface (and more importantly the same number of high speed serdes pins)