The practical reasons... To create chaos and force recruits to function with in that chaos. Combat is noise and chaos. Also, combat leaders to "train" their voices to handle constant, high volume yelling. In order to lead in battle (specifically in small infantry units) you have to make your voice heard over the din. Try screaming without any practice. You will at least be hoarse, if not mute the next day. Now try that day after day in combat.
Jack Rabbit wrote:
There is a very important psychological component to the yelling at basic training. It tells the recruit that the DI is in charge. Basic training is just that: learning in the shortest amount of time possible to become something useful. Yelling creates a very high stress environment which is conducive to learning quickly, it forces you to react and function under stress which may letter become functioning under fire. The recruit has to learn to obey immediately and without question every order with exactness. The DI isn't mad, the yelling is part of the indoctrination process.
this^^ I couldn't join due to medical reasons but I really researched the services.