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It is the same as a bolt gun - making the locking lugs all seat fully.
You must not really understand what holds an AR barrel in place if you don't think an un-square receiver face can change the angle of the barrel. Gotta shake my head at that comment. What else do you think sets that angle? It's not the bore in most cases, they're too loose unless you're using something like a BCM thermofit upper.

Bolt guns lock the lugs against the steel receiver, the barrel is threaded into the receiver.

AR's lock the bolt into the barrel extension that is threaded into the barrel.

Have you ever seen how much play the bolt carrier has in an upper receiver?

How about the play the bolt has in the carrier?

If anyone wants to square up an upper, be my guest. I would simply like to know if anyone has took a gun that wouldn't shoot and squared the upper and it started shooting bug holes. Anyone?

Been doing some reading on the subject and a few things stood out that I haven't thought about.

In the days of $39 uppers, tolerences can be all over the place. You may very well end up with one so out of spec, even the tool won't fit down inside.

If you can true it up in a lathe after centering on the rails is the best way, otherwise it is possible to apply to much pressure the wrong way.

Some claim that your sights will be centered. Others said it's easier on your bolt. Others said it was a solution looking for a problem. Most said it doesn't hurt a thing but not a single case of "it turned my blaster into a hammer". Most cases the amount of material removed is about 1/1000". I have a stripped upper from Rainier Arms here and pulled a carrier out of my Colt 6920, the carrier to receiver rails moved up and down and sided to side well over 1/1000", so did the bolt inside the carrier.

Now that said I will relay a story told to me by a friend who had a rifle that shot well but not great. He knows a famous shooter/gunsmith and they started talking about said gun. The guy took it and turned it into a great shooter. My friend said he didn't see what he did but he told him the bolt was not square so he lapped the bolt. Maybe he lapped the upper? I truly don't know, he just said he lapped the bolt.

I still say there is too much tolerance in any upper/carrier fit to make a difference in most but I'm gonna say if done right I suppose it won't hurt a thing either.

TWR ; I can see where you are going with this, I would also like to see some scientific proof of the results of squaring the Upper. The problem with trying to prove it would be finding someone with a large enough supply of rifles that they would be willing to test before and after. Maybe you could start a experiment and have members sign up and start an experiment. Post their groups ,square the receiver using the same method with the exact same tool and then test again.

Square an upper? Naw, just buy a BCM.

https://www.rainierarms.com

Sky's the limit anything you want, at some of the best prices and great guys to work with.