Sometimes you have an object that already does what you need to it to do, for example the GTK+ library.
Now, you come along and already have this great app that interfaces to Windows using another class.
Now you want to port that program to GTK+, you write a small wrapper around GTK+ that implements the functions required, and swap out the Windows class with the GTK+ class, and now your program has GTK+ as it's window drawing tool, and the class essentially is like a wrapper around a candy bar. It protects and provides a clean way to access the underlying candy!
