The reason becomes more apparent when you have to maintain a larger project. When a method or variable is public, you have to be careful when you make changes to it, because you never know which parts of the codebase rely on its exact behavior.
But when a variable or method is private, you know that it is not used outside of the class. That means there is a lot less code you have to pay attention to when you make changes.
By making class features private and public, you clearly separate the interface to the outside world from the internals. The less you exposes to the outside world, the more freedom you have with what the internal implementation does.
When you, for example, always make variables private and accessed them through getters and setters, you can later change them from a variable to a computed value, and then even later add caching to the computation for performance reasons. When it would be a public variable, you would have to change code everywhere the variable is used. But when you expose it to the outside world through getters and setters, all other code can keep using the class as if nothing had changed.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
