Visibility: public/private/protected

When discussing OOP, we must touch the "visibility" topic, meaning the public/private/protected keywords before properties/methods. When to use which?

For example, when creating methods in Controllers, we're probably used to "blindly" type public function ...:

EveryControllerEver.php

public function index()
{
    // ...
}
 
public function create()
{
    // ...
}

Question: in what cases would we change the methods to private or protected?

First, let me tell you...