At GVA some flights are remote and use busses. I remember back to when I was crew in some stations you'd get an air bridge and a few minutes later the rear doors would be opened for disembarkation via steps and other times it would open just so you can out the rubbish out. What I'm trying to say is you never know what's going to happen.
So if it were an air bridge then possibly they are saying it just in case as you are not necessarily informed of whether the rear door is to be opened or not and when you're saying bye to pax you won't know the rear door is opened necessarily as the indication panel is out of view.
It's probably just back covering just like how you're always asked to put your seat upright on every flight despite most EZY seats now being non-recline.
Anyway, we're talking about a 30 yard walk to the terminal. I'm sure it's not the end of the world if we need to put our phones down for the seconds that it takes to walk.