Just as an example. You are gay. You live in a gay friendly place. Until one day, a new government takes office that will incarcerate gay people. They now have access to every one's phones who previously had nothing to hide.
Everyone has something to hide... unless there's something utterly wrong with them.
For the same reason you close the door when you are pooping on the toilet. You know what you are doing there, I know what you are doing there, so leave the door open.
That's what I always lead with, if we come into that territory :)
The GP argument is the correct one. You might not mind now, but that's not the problem.