Like what Dreamora says :-)
The reason being, that because the nitty-gritty code is hidden away inside the Type, you can, at a later date, completely re-write that part of it without having to change code outside of the Type.
Otherwise, can you imagine the chaos of having to change, say, 7 areas of your program, because your List is now implemented in a different way?
You want to make your life as easy as possible, after all :-)
The reason being, that because the nitty-gritty code is hidden away inside the Type, you can, at a later date, completely re-write that part of it without having to change code outside of the Type.
Otherwise, can you imagine the chaos of having to change, say, 7 areas of your program, because your List is now implemented in a different way?
You want to make your life as easy as possible, after all :-)