I’m just a one-stripe white belt, but my understanding of the guard for self-defense is that it is best position you can be in IF you find yourself on your back.
For punch prevention, a closed guard allows you to manage the distance. Hip them out and they aren’t reaching your face face with their fists. Hip them in and hold them close, no room is available to punch.
From the guard you have sweeps available, which can put you back in mount or side control.
If your guard is closed you won’t be getting a knee in the groin. I don’t know a lot about open guard yet, but I’d be trying to grab their sleeves or wrists, get a foot in their hip and start kicking them in the face, should the opportunity present itself. I’ve never had someone try to knee me in the balls when I’m playing open guard, but now I’m kinda curious for someone to try.
I wouldn’t ever pull guard in a street fight, but my guard game is very rudimentary. Even so, if I’m on my back guard is the best place to be. I don’t have a lot of attacks from guard, but I have a decent idea of how to manage distance and minimize damage. It can also be a stepping stone to a better position.
Long story short, I’d always prefer to be in mount, side control or on a person’s back over them being in my guard during a street confrontation. Standing would be preferable for me too. The guard is there for options if I find myself on my back.
I’m just a one stripe white belt though. This is just my $0.02.