

In addition to corsicanguppy’s comment, some — often important — programs actually expect the system to be secured in a particular way and will refuse to function if things don’t look right.
Now, you’d be right to expect that closing down permissions too tightly could break a system, but people have actually broken their systems by setting permissions too openly on the wrong things as well.
That said, for general, everyday use, those commands don’t need to be used much, and there might even be a way to do what they do from your chosen GUI. Even so, it nice to know they’re there and what they do for those rare occasions when they might be needed.
For me it’s a vowel sound, sure, but the mouth is definitely in, or moves to, the position it would be if a pronounced R, and presumably a vowel, was about to follow.
Probably more so than in the pronunciation of name of the letter R itself, in fact, which is indistinguishable from “ah”.
But perhaps more importantly, no, we generally don’t say “brr” anyway, except in exaggeration or for effect. You’re more likely to hear something like “it’s bl–dy freezing” or “my hands/feet are like ice”.