When an artist is working with the pen, the touch screen has (in the past) turned touch off to keep from accidentally pressing buttons with the part of the hand the rests on the screen while drawing.
It’s easier to understand envisioning cooking in the kitchen or a mechanic working on an engine, the bulk of “real” work happens at the stove and under the car. With a second person to gather supplies, tools, parts, etc the work can be done fluidly and rapidly.
The old way of using the pen and touch was akin to working alone. Simultaneous Pen and Touch hotkeys is that second person assistant to the artist.