With my laptop in my lap, I typically have to be leaning back to comfortably use the keyboard. Sitting up or slightly leaning forward, it's more comfortable to grasp and scroll with the screen itself, maybe bent a little further back than typical usage. It's a more comfortable resting position.
Although sometimes I just flip my yoga into a tablet and that's just swell
more than a page turn?
I find the continual swiping distracting. And it's a much more complex motion (akin to a page turn) and must be done more frequently than paing. A tap of the space bar is about the simplest motion, and (except in cases of bad page design) gives a clean "start at top for next page" rather than trying to figure out where on the screen your next line is.
Preferences vary, obviously.
Yes, a page turn with a book gives you a very specific place to continue. Spacebar or page down doesn't.