I'll use a simple example that you might encounter in a game:

class Weapon and class Tool. Weapons are used for attacking enemies, and have properties like damage, range etc. Tools are used for exploring the world: prying open lockers, breaking down doors. They have properties like weight, lifespan, etc.

So what should a Crowbar extend from if the game rules say it can be used as both a weapon, and a tool? To add even more complexity, what happens when you have class RangedWeapon extends Weapon and class MeleeWeapon extends Weapon? The crowbar is both. You can throw it, or beat people with it. So even if you decided it should extend from Weapon, which sub-class of weapon should it extend from?

Instead, it would be better if you could combine different behaviors together. Combine RangedAttack, MeleeAttack, Leverage, and Smash behaviors together into a new game element called Crowbar. There is no inheritance tree or taxonomy to worry about - only discrete behaviors which can be combined in different ways to form new weapons/tools/gameplay elements.

@ron_man This is something I'm trying to learn more about as well. I've been exploring the Elm language, which is a purely functional object-oriented language and it has opened my eyes to some of the benefits. Essentially as I understand it, if your objects/functions are composable they are not reliant on one another.

However if your object extends another, it is now reliant on that previous objects API -- class A extends class B. Do this a couple more times and you've got class D extends class C (which extends class B, which extends class A. What if I make a change to class A? It starts to get hard to figure out if that's going to break the other 3 classes.

With composition I can say class D is composed of class A, class B, class C. It doesn't inherit from any of them, it is composed of each of their unique structures.

I hope this is a good explanation - I'm sure it's one that could use some help/fixing as my understanding of this concept is still growing as well.