HDRP 2019.1 and it's post proccessing

I see a lot of tutorials making a separate volume for the pp instead of putting it on the scene settings volume, is there any reason for this or are they just doing it to keep the screen clean from other stuff?

edit: how do I edit the title?, there shouldn't be a ' in the it's.

 

recently i switched from 2018.3 HDRP to 2019.1 HDRP and i saw theres 3 different volume settings with different depth orders. and every one works with them in a unique way.also theres some unoptimized features like screen space reflection and a limited version of planar reflection probe (limited to 2 .idk why ?! because even if your gpu could handle that ,unity wont allow you).
so i guess This HD render pipeline is still going through a lot of changes .personally decided not spending more time on it till its production ready.