The reason is along the same lines as the "scant" quarter-inch theory.
The thread takes up space.
So if you just sew them and press them (open or to the side), you're not getting it as flat as you can because there is a bit of a bump there from the thread.
Can you see it from the space shuttle?
No.
But try it. Take two scraps and sew them together.
Feel the thread line.
Now just take you iron and press it on the seam line.
Feel again. You can feel how it's nice and smooth.
So now when you go to press the seams (again, open or to the side depending on your preference or situation), it will be flatter.
Also, when you go press the seam to the side, you'll get less tucks and pleats. I just find they move better when the seams are set.
So set the seams, press, and this is where you can bring starch in if you want, press again.
I like my seams and blocks to be as flat as wallpaper.
As for time spent, it's negligible. ;-)