Access modifiers are treated in K&B chapter 1 and any
java tutorial book.
Practically: Think of a helper method of a public method. If the helper method would be coderanch,too, everyone could use it (if you like or not). If you then want to change the parameter list of your helper method, for example, other classes may be affected. But if your helper method is private, nobody else can use it and you don't have to care about any other classes. So your system has less changes and becomes more stable.