It's a co-processor, used for applications that benefit from the features it has the Intel GPU does not. It is not a true discrete GPU -- it has no physical connection to the display panel. It's a render-only device -- the Intel GPU passes all video data to the display.
Most systems are (and have been for some years now) designed this way. There are some high-end gaming systems or high-spec models (the XPS 9700 with the RTX GPU is one; so are many Alienware and some high-end Precision systems) that do have firmware-controllable outputs. The same is true of competing systems.