Do I need to learn Programming Concept and Modelling?

I'm a Freelance Web Developer and I want to ask if I need to learn Programming Concept and Modelling. Because I've never been in a corporation and I want to extend my knowledge as a Web Developer. So I have this thing in my head that "Do I really need to learn this concepts and modelling?". I can code/make a software though, In my experience I want to learn advance programming languages like other frameworks but every programmer need to know the basics or foundation of what he/she is doing, because all of those processes are interconnected with concept and modelling. And every programmer have different strategies in making a software.

1.) So where do I start if I want to understand clearly this Programming Concept and Modelling?

2.) Do I really need to include Modelling like waterfall model in making my proposal to establishments or prospective clients?