A popular reason to consider training is for house training the dog. It has been noted that the majority of dogs will not go to the bathroom where they sleep and using a crate helps to train the dog to hold their urine till they are free to go outside. The more you use the crate, the more in habit the dog gets of waiting and only going at the right place, helping to prevent accidents in the home.
A new puppy, exploring his new surroundings, can get into a lot of trouble as well as danger. Until the puppy is familiar with the house and trained about certain do's and don'ts in the home, a crate is often used. This ensures the safety of the puppy whenever you can't keep a close eye on him for whatever reason.