What's the reason for squared off trigger guards?
So people who want to have something to grab on to. The advisability of doing so is debatable.
there is another point not mentioned. The squared off trigger guard give a (relatively) sharp "point" when the pistol is used as an impact weapon.
It's almost never talked about (outside of specialist discussions) because we all feel it shouldn't need to happen (because we should shoot and stop the attacker) but real world gunfighting still sometimes does include using the gun to hit someone, not just the bullets. its a last ditch, desperation thing, usually, but it can still happen.
So, its a feature some people think they need and use, and others don't, and don't use, and it has a feature no one wants to need to use.
personally, I don't use it, have tried, simply not for me. I have a couple of Sigs with it, I just ignore it, and, as far as I can tell, my shooting hasn't suffered because of that.