I really don't understand how someone can be only a front end developer or a back-end developer. Why would anyone want to learn just one thing when the thing that is valuable is delivering actual applications. Who wants to have to depend on someone else to accomplish anything and make a living?

To answer your question, a front-end or back-end developer is someone who (erroneously) sees development as sort of blue collar labor like a plumber or a electrician. A web designer is someone who decided they could just do all the easy, fun stuff while getting to decide everything and prefers to work with trade-type developers for the un-fun bits.

Object oriented scripting is important because without some sort of organization of code, you end up with a huge mess of code and variables. Without encapsulation you never know when and how it's safe to modify data or what side effects it might have or what code is allowed to modify what data.