I am wondering and have been, why would copywriting be beneficial to a web master? I don't really sell products so dont see a need to learn it. But am I wrong? What could I use copywriting for?
well-written copy always reflects well on your brand, regardless of whether you have something to sell or not.
At some level, everything's about being able to sell yourself or your website. Even if you have a simple website about raising Labrador puppies, you'll do better if you can convince/sell your site visitors that you actually know what you are talking about.