All of the previous writeups on this subject only address the cosmetic appeal of shooting a gun sideways, as seen (as mentioned above by amnesiac) in John Woo films and gangster movies. These points are all valid; the gun is more likely to jam in this case, IF it is being fired right handed, in which case the ejection port, which is on the right side of the pistol, will be facing upward.
If, however, one is firing left handed, the ejection port will be facing downward and the possibility of jamming will be greatly diminished, perhaps even less than if the gun is fired in the traditional, zero degree alignment. Also, if firing a revolver, jamming is not an issue.
With that out of the way, we can address some tactical/functional reasons why you might actually want to fire this way. Note, since you will not be able to use the sights properly you should only use this technique at close range where you only need to point the gun in the general direction of your target- maybe fifteen feet or less. Point aiming, as the old gunslingers did it, can only be learned through practice so that the gun becomes an extension of your hand, and you know by feel where the bullet will go without having to use the sight posts. Practice makes perfect.
For one thing, the recoil of the gun will be absorbed by your pectoral muscle, instead of just the muscles of your triceps and frontal deltoid. Therefore the recoil will be marginally easier to control. And when firing a machine pistol, such as an Uzi, MAC 10 or Heckler & Koch MP5K, the recoil of the gun will propel it sideways in a vertical sweep. This is very useful for putting many bullets into a whole roomful of multiple assailants, a situation that calls for quick life or death thinking, and emphasizes gross motor skills over fine motor skills. Just make sure that you aim at the guy on the far right first, hold down the trigger, and let the recoil handle the sweep for you.
Also, when police or soldiers are firing form a tight squad formation, they might fire with their weapons turned sideways so that the hot brass cases are ejected downwards rather than into their buddy's shirt.