I think that’s the key, not just ‘‘sketching’’ but doing it with a purpose. Taking a bit of time, analyze the result and refine it. Just sketching with a purpose will improve your skills, but I think doing it this way you’ll also improve at designing.
At our company we have a good balance between sketching/cad/prototyping, definitely you can see how your skills improve on all levels. I particularly enjoy sketching the most, so I try to spend some time sketching at home as well. For me the key is finding a sketch I really like on tumblr or pinterest and trying to replicate that style. Cars and shoes are fun to sketch because you can do pretty much whatever you want and there are tons of styles and examples out there.