Another analogy: let us say you are a developer and whenever you like you order computer science books from the market directly - you know the sellers and their prices. In fact your company might have a preferred seller and you contact them directly. All this works fine but may be a new seller is now offering better prices and your company wants to change the 'preferred' seller.
At this point you have to make the following changes - update the contact details (and other stuff) so as to use the new seller. You still place the order directly.
Now consider we introduce a new step in between, there is a 'library' officer in the company and you have to go through him to get the books. While there is a new dependency, you are now immune to any changes to the seller: either the seller changes mode of payment or the seller himself is changed, you now simply put an order to the librarian and he gets the books for you.
