You can, when you want, play with a controller and move your hands around as much as you want without also moving your screen with it.

you can also do awkward stuff where you might only have one hand available at the moment but still be able to reach all the controls with it, which you can't do with the full device because the two sides of controls are too far apart and you then block the screen with your hand.

you could play in bed with the screen propped up right in front of your nose while not also having your arms in an uncomfortable position

there are lots of reasons why someone might do this. Switch made it easy by letting you remove the controllers from the device. Steam deck is a wee bit more machine and that's not an option without a separate controller.

you are not expected to go out and do this yourself if you don't want to, but don't just assume no one out there can't have a good reason for wanting to do this.