No matter how experienced you are, sitting down to do a drawing study once in a while is always going to teach you something new. As you develop your skills to higher levels, the bar is also raised on the levels of learning you still have yet to get through.
While style constantly evolves on it's own, it's important to remind yourself of the subtler observations real world studies have to offer. They help you to inject that added level of believably and relatability into your work. As comic book artists we should constantly be aware of how people really look and really interact within our environment.
What I really enjoy about a solid study session though is how it trains us to translate a real-life photo ref into a stylized comic book illustration. There's so much insight to be had in the subtle tones formed by unpredictable lighting conditions, the flow and energy of the pose, as well as the subtle variations in anatomical structure from person to person.
This is a quick study of Milla Jovovich that I drew up the other night. She's one of the actresses I typically reference when it comes to capturing the look of my female characters (among others). Something I got out of this study in particular though was the balance of detail and simplicity in the background. It's always tempting to become overwhelmed by backdrops, but you can often get away with relatively few elements being there and still having it establish a great mood that compliments the character.
Now... go grab yourself a sketch book and fill it up with some studies of your own. You'll be blown away by what it'll do for the development of your drawing skills. Good luck! ;)