They usually mandate that we wear them when we're ground guiding vehicles or guarding roads while in garrison so that headlights from cars reflect off of them. During the day they are pointless in my opinion.
What? No, it's not. Every member of the royal family must complete military service. Both Harry and William have done time in the army. Elizabeth II was an ambulance driver in WWII.
He's behind the wire in this shot, meaning he's required to wear the reflective vest to make him more visible to people doing things like operating heavy machinery. Because, you know, camouflage works sometimes, just not when you want it to.