[Double Naught Spy]

They get rotated for a couple of reasons dealing with making it easier to get a new magazine into the gun. The first reason is to gain better access to the mag release button. Unless you have really long thumbs or big hands, the mag release is really difficult to reach for some people. Rotating the gun gives you better access to the button.

About 7 or 8 years ago, I had a class with Ken Hackathorn who advocated holding the rotated gun in front of one's face because at that moment, the most important thing in the world (if you are in a battle) is getting the gun reloaded and back into the fight. So you want the gun up in front of the face so that you can clearly see the mag and magwell such that the insertion isn't fumbled in any way. So rotating the gun gives you easier access to the mag well. Holding the rotated gun in front of your face lets you see the process. Personally, the notion of holding the gun in front of one's face and losing situational awareness is an awfully shortsighted idea.

Keeping your target covered during reload, toward the threat, or keeping the gun pointed safely down range really does make much more sense from the perspectives of safety and defense. It is a bit more difficult to become proficient doing in that manner, however. If the problem is just getting access to the mag well, then after dropping the spent magazine, the gun can be rotated along the axis of the bore such that the gun remains pointed down range and toward the target/threat.