Imagine doing a rough shore entry, you just cross the surf zone, and a shell found its way into your reg making it freeflow. A removable faceplate is the difference between packing it up and going home or having an awesome dive.
#1 reason to always have removable face plate!!!
I've had sand get all up in my diaphragm many times when squeezing through a cave restriction. It's nice being able to open it up and clean it out mid dive without having to stop or end the dive to fix something.