"According to Islamic tradition, the animal is brought to the place of slaughter and laid down gently so as to not injure it. The blade must be kept hidden until the very last moment while the jugular of the animal is felt. The conventional method used to slaughter the animal involves cutting the large arteries in the neck along with the esophagus and vertebrate trachea with one swipe of an non-serrated blade. Care must be taken that the nervous system is not damaged, as this may cause the animal to die before exsanguination has taken place. While blood is draining, the animal is not handled until it has died. If any other method is used its meat will not be halal."
And with the non-halal method, if in the animals contain some bacteria or virus in their brain, then it can spread from blood vessel to all the part of meat...and when you eat that meat, the virus and bacterias can spread to your body too...
When the blood is drained from the animal all the uric acid and other impurities found in blood are also drained. High uric acid consumption in meats is a primary reason for a debilitating disease called gout.