Finishing the rise

I'm making these for the first time and they are adorable! I was wondering why, when finishing the rise, we are instructed to clip the seam between the notches and simply press open the rest of the seam, rather than finish the entire seam (steps 7 & 8 of Assemble the Pants Panels)? I'm a new sewer, so I'm inquiring just for my own edification. Thank you!